There are a lot of conspiracy theories out there, and while most of them tend to be, well... stupid, for the most part they're pretty harmless (although some of the people who believe in them are not so harmless).
On the other hand, there are some conspiracy theories that can drive a person to become paranoid, and possibly even act out in very disturbing ways, perhaps even in a violent manner towards those who they feel are apart of that conspiracy.
Here are a few of those conspiracy theories:
Chemtrails is a conspiracy theory in which some conspiracy theorists believe that the government is spraying chemicals on the population from air planes, and that the contrails coming out of many air planes are actually laced with chemicals, and thus called "chemtrails".
Now despite the fact that the chemtrail conspiracy theories have been debunked, some people take it very seriously. So seriously in fact that many conspiracy theorists will spray vinegar into the air whenever they see a contrail (because they believe that doing so will destroy a chemtrail), or even go to a hospital whenever they see a bunch of contrails in the air, because they are seriously afraid it might cause them health problems.
Some people have even taken their paranoia a step further and have threatened shoot down air planes because they think they are spraying chemtrails.
If there is one conspiracy theory that believing in can cause a person to become very paranoid (although it may also be a strong indicator of a serious mental illness) it would be the mind control conspiracy theories, particularly the ones that involve some sort of telepathy.
The fact is that if a person believes that their mind can be attacked at any point in time it tends to leave that person extremely paranoid, and causes them to do some pretty bizarre stuff, such as wearing hats made out of aluminum foil, or even covering their entire house in aluminum foil (because they believe it will block out what ever rays are being used to control their minds).
This may also cause some people to be wary of other people as well, even people who might try to help them overcome their fear, because they fear that person might come under some kind of mind control as well and harm them.
Often times attributed to David Icke, there are some people out there who seriously believe that there are shape-shifting reptilian aliens that not only walk among us, but are in control of the world, and that many world leaders are actually aliens from either another world or dimension.
I really wish I was making this up, but sadly I am not. There are people out there that are so paranoid, and so delusional, that they seriously believe that the leaders of the world are actually shape-shifting lizards from another planet or universe.
The belief that another person is a shape-shifting alien doesn't just include world leaders, it can actually include anyone, be it a celebrity, a rich person, some random person, myself, and even David Icke (who has been accused of being a shape-shifting alien).
Some people who believe in this conspiracy theory are even so paranoid and delusional in their beliefs that they believe that they themselves are a shape-shift alien...
Just about every skeptic and debunker has at one time or another been called a dis-info agent. This is just a general reaction by conspiracy theorists when a skeptic either refuses to believe them, or presents evidence showing that the conspiracy theorist's claims are false.
While some conspiracy theorists may cry out "dis-info agent" whenever they encounter a skeptic, many conspiracy theorists really do believe that there are dis-info agents out there, and that they are out there to "trick" people into no longer believing a conspiracy theory that they promote.
This belief in dis-info agents often times causes many conspiracy theorist to disregard any fact that would disprove their conspiracy theory, and basically regard it as being nothing more than disinformation, no matter how sound it is. This paranoia can even go so far that some conspiracy theorists will believe that other conspiracy theorists are also dis-info agents as well if that conspiracy theorist's conspiracy theory is to much un-alike their own.
FEMA camps is another one of those conspiracy theories that has been around for a long time, yet has no evidence to back it up, but many people still believe in it and that these FEMA camps exists.
People who believe in this conspiracy theory claim that the government is constructing prison camps to be run by FEMA all across the country inorder to imprison anyone who they feel will oppose the government in some up and coming police state (which according to them is just around the corner... and has been for the last couple of decades).
This constant fear that the government is going to create a police state and ship people off to these imaginary prison camps can cause some people to act out in very irrational ways, including building huge bunkers in which to hide out in and perhaps fight from, buy up huge amounts of guns and ammunition and other type of supplies, and cut off contact with people. It can also make a person become violent towards anyone who works for the government (especially law enforcement), or just drive a person to the point of suicide due to the fear that they will be imprisoned or killed.
There are many people out there that believe that, despite the evidence to the contrary, that the Illuminati exists, and that it controls the media, and the banks, and the government, and corporations, and just about everything... except apparently for the internet where anyone can create a website that opposes the Illuminate, as well as whomever is broadcasting Alex Jones's radio program.
The belief that the Illuminati exists can be very strong in some people, and can cause some people obsessively look for "Illuminati" symbols wherever they believe them to be. It can also cause them to accuse people (mostly people who are rich or famous, or are a politician) of being a member of the Illuminati. Sometimes it's not even someone whom is one of the rich and powerful, sometimes it can be just a regular person whom is apart of a group that is accused of being a front group for the Illuminati (example: Free Masons), or even a certain religion.
Because this belief in the Illuminati can be so strong in some people it can actually cause some people to distance themselves from anyone whom they believe to be a member of the Illuminati, or get into huge arguments with people who don't believe in the Illuminati.