Transporter accident
A transporter accident or transporter malfunction was an often unintentional occurrence caused by the malfunctioning of a transporter, however, some transporter accidents came as a result of intentional causes, such as murder or assassination.

As with all technology, along with the unknown risks associated with matter-energy conversion, transporter accidents were often an unexpected reality of the 22nd, 23rd and 24th centuries. Though transporters were a relatively safe way to maneuver from one point to another, there were nonetheless multiple cases of transporter accidents. 

Numerous independent technological and or environmental influences can cause a transporter malfunction. Defective technological components, such as bio-filters, pattern buffers, and molecular imaging scanners, and likewise, anomalies, power overloads, malfunctions were often investigated. Scans of a planet's surface or atmosphere were also conducted, as well as the consideration of alien interferences. ()

By the mid-24th century, there were typically, on average, two or three transporter accidents a year across the Federation, yet millions of people were transported every day. Because of transporter accidents, some people suffered from transporter phobia or experienced transporter shock. (; )

Physical and mental manipulations

Death 
In 2139, the pattern of Quinn Erickson, son of the inventor of the matter transporter Emory Erickson, was lost during testing of new sub-quantum transporter technology. However, the pattern was preserved in a subspace phenomenon known as "The Barrens". Fluctuations in subspace allowed Erickson's pattern to reform periodically, taking the form of a ghostly figure. When Erickson was finally rematerialized in 2154, the pattern had deteriorated, and Erickson died seconds after materialization. ()

According to Montgomery Scott, as of 2269, if a transporter had insufficient , then there was the real possibility that transportees would "come aboard [as] a mass of dying flesh." ()

In the 2270s, two  crewmen, including Science Officer Commander Sonak as well as a female officer, were killed in a transporter malfunction while beaming to the Enterprise, which had recently been refitted. Sonak was coming aboard to initiate his duties as the ship's senior science officer. During the transport sequence, a malfunction caused a corruption of the buffer pattern for the commander and the crewperson. As they began to re-materialize, the transporter systems couldn't cope with the data loss and their physical form became "deformed"; the female crewperson screamed out in agony. The transportation failed, returning the deformed bodies back to Earth. The transport chief planet-side reported back, shaken, "Enterprise. What we got back didn't live long... fortunately." ()

The Bajoran Trentin Fala was killed in a transporter malfunction aboard one of Deep Space 9's runabouts that was caused by a remat detonator. The detonator caused a power surge in the pattern buffer, and interfered with the integration matrix. Lieutenant Commanders Worf and Jadzia Dax, who were conducting the transport, attempted to transfer Fala's pattern to the secondary buffer while boosting the gain to the energizing coils, but the settings were maxed out, and the transporter beam was scrambled as she rematerialized, leaving a smoking skeleton wearing tattered clothing on the transporter pad. Though the transporter security system typically scans for such devices, the culprit responsible for planting the detonator on Fala had a sophisticated understanding of Starfleet security protocols. ()

In early 2375, Weyoun 5 was killed in a transporter accident. Damar was to have been with him but "had been called away," casting suspicion on him for possibly "arranging" the incident. ()

Reverse aging

In 2369, Jean-Luc Picard, Ro Laren, Guinan and Keiko O'Brien were physically reverted to twelve-year olds after the transporter deleted rybo-viroxic-nucleic sequences from their genes. This presented some difficulties, notably with Picard's ability to command and the O'Briens' marital relationship. There were two options: do nothing or attempt to recreate them using their last fully-formed patterns to replace the missing sequences. Beverly Crusher was hesitant to send them through the transporter again, until they could figure out what had caused the malfunction, fearing they would lose even more and become younger.

Later, it was discovered that a molecular reversion field had penetrated the hull of the shuttle, causing the transporter to register only part of their patterns. Following the upload of their adult patterns, Miles O'Brien successfully re-aged Picard and the others. ()

A similar process was also used to restore Doctor Katherine Pulaski to her original age in 2365. ()

Foreign object contamination

In 2151, Crewman Ethan Novakovich was beamed back from the face of a planet later known as Archer IV by the still-experimental transporter system aboard . The emergency transport was attempted during a fierce windstorm. Upon arrival, he was unconscious and had rocks, leaves, and other debris from the planet's surface embedded in his skin, due to a malfunction in the phase discriminator.

However, due to the fact that "Human skin is a resilient organ," Phlox was able to remove the debris and repair the rather extensive damage, and Novakovich was expected to recover fully. ()

Transporter psychosis 
In early models of the transporter, errors at the molecular level during rematerialization could cause serious damage to living subjects over time. As a result of these errors, some subjects developed a syndrome that was named "transporter psychosis", first diagnosed on Delinia II in 2209. This was eliminated after multiplex pattern buffers were perfected, around 2319. ()

Phasing 

In 2380, Ensign Sam Rutherford experimented with the transporter systems aboard the , with the hopes of making the transporter itself a bit faster. He tested his work by transporting Ensign Brad Boimler to one pad and back, but an error in his work resulted in Boimler beaming back one millicochrane out of place. This resulted in Boimler appearing out of phase, and exerting the loud pitch of the transporter itself. While this accident warranted Dr. T'Ana to contact Division 14, and give Boimler a small leave of absence to recover at Endicronimas V's medical spa, colloquially known as "The Farm". Boimler was unharmed, as the effect itself was cosmetic in nature, and wore off after a while. Later, at the behest of Ensign Beckett Mariner, Rutherford ended up using the same modifications on the Cerritos main transporter platform to save the entire crew of the . The crew materialized with the same phase effect. ()

Splits and splices

The "Yin Yang" effect 

On stardate 1672.1, in 2266, a strange ore altered the function of the transporter, causing a transporter accident in which Captain James T. Kirk was split into two separate entities. One man embodied all of Kirk's so-called "positive" qualities and the other embodied all of his "evil" qualities.

It was some time before the mishap was discovered, and the malignant version of Kirk roamed the ship, stealing brandy, assaulting crewmen, and even attempting to rape Yeoman Janice Rand. When he was cornered and finally captured in the engine room, the transporter was further damaged by an errant phaser shot he fired. Scott and Spock isolated and repaired the damage.

Their repairs were confirmed when a test animal, which had previously been split in a similar manner to Kirk, was sent through the transporter in an attempt to reintegrate the two creatures. Upon reintegrating, it rematerialized dead, but McCoy speculated that this was the result of the animal not understanding what was happening to it and dying of fright, whereas the sentient and rational Kirk would be able to understand what was being done to him and thus be able to cope with it.

Though he was crippled with indecision, Kirk was able to make the trip (albeit barely), and his two halves were reintegrated once again. ()

Split one entity into two identical entities 

In 2361, another transporter accident resulted in William T. Riker being divided into two separate Rikers. () Unlike the two Kirks created in 2266, both Rikers were functionally identical to the original man. (; )

The incident occurred on Nervala IV, while the  was conducting an evacuation of a science outpost on the planet. At the time, William T. Riker was serving as a lieutenant in Starfleet and was part of an away team. An unusual distortion field meant the Potemkin had difficulty beaming him up. A second confinement beam was initiated to overcome these difficulties, with the intent of reintegrating the two beams in the transporter buffer. This was unnecessary, as only one beam was successful at transporting Riker; the modulation of the distortion caused the second beam to be reflected back down to the surface, materializing the two Rikers, one on the ship, and one on the planet's surface.

The Potemkin left orbit, unknowingly abandoning the duplicate Riker. After eight years, this accident was discovered by the Enterprise-D which revisited the planet, found the second Riker and brought him back to the ship. ()

Twenty years later, in 2381, history repeated itself when Lieutenant junior grade Bradward Boimler, on a convert mission to Karzill IV, found himself duplicated.

While he and his away team were besieged by Pakleds, Boimler used his knowledge of Riker’s splitting, to take down the distortion field keeping them from being beamed back up to the . However, the field came back up before Boimler could be beamed up and a second containment field was used to strengthen the beam. This resulted in a clone being created, which was confirmed when the other Boimler returned to the Titan soon after via shuttlecraft.()

Two entities merged into one (aka "splicing") 

 
In 2263 of the alternate reality,  chose to beam  and  onto the USS Franklin separately, via the cargo transporters, saying he didn't want to run the risk of splicing them together. McCoy found that prospect distasteful. () 

Lysosomal enzymes of an alien orchid were the cause of such an accident in 2372. Tuvok, Neelix, and the orchid were temporarily merged into one being during transport; as the orchid aided microscopic entities in breeding by allowing them to combine with each other, it accidentally caused Tuvok and Neelix to combine when they were broken down into atoms during transport. "Tuvix", as he named himself (or "themselves"), was a complete mixture of the talents of both crew members. 

After discovering how to separate the two patterns and retrieve both Tuvok and Neelix, Tuvix protested that such a procedure would be equivalent to murdering him, but the procedure was undertaken anyway, and Tuvok and Neelix were restored. ()

Unintended destinations

Incorrect coordinates

In the 2270s, the programming of the Enterprises transporter by Ari bn Bem caused Kirk and Spock to re-materialize just next to a cliff overlooking a water party, the rest materializing safely on the cliff. Once materialized, the captain and first officer proceeded to fall into the water. ()

Mirror universe

In 2267, an ion storm near the Halkan homeworld resulted in a power surge in the 's transporter, causing momentary interdimensional contact with a parallel universe. Captain Kirk, Doctor McCoy, Commander Scott, and Lieutenant Uhura, who were beaming up to the Enterprise at the time, materialized in the other universe, transposing with their counterparts from that universe, who experienced an identical accident at the same time. Later, after reviewing the events which led up to the accident, the Enterprise crew members were able to recreate the power surge using energy tapped from the ship's engines, and return to their own universe. ()

According to Intendant , all transporters in the parallel universe were subsequently redesigned in order to prevent a recurrence of such an event. Despite this, Terran rebels in that universe were able to develop a multidimensional transporter device capable of reconfiguring transporters for use in beaming from one universe to the other. ()

Time travel
In 2371, Benjamin Sisko, Julian Bashir, and Jadzia Dax were accidentally transported to the year 2024 when the chronitons generated by the Defiant's cloaking device were contaminated and enhanced by an explosion in a microscopic singularity, while it was passing through the solar system at the time of the beam-out.

Fortunately, Miles O'Brien was able to devise a way to use the residual chronitons to send Kira Nerys and himself to different periods of Earth's history to try and find out where the away team was sent, and then bring them back home.

However, while in the 21st century, Sisko accidentally caused the death of Gabriel Bell, forcing him to assume the identity of this historical figure. () As a result, Sisko is actually in all the historical photos of Bell. ()

 
Events
Death