The short answer is: the U.S. has probably not helped ISIS inadvertently, but arms have probably fallen into ISIS hands from the U.S.
In essence, you are proposing two different mechanisms for the delivery of material.
The first is the acquisition of arms from the U.S. consulate in Benghazi, and the second is accidental delivery of weaponry intended for anti-ISIS or anti-Ba'ath fighters in Syria.
The first mechanism is highly improbable.
First, consulates are not usually armed, and the one in Benghazi was specifically understaffed and under-protected, meaning that very little was available to be shipped from Libya to Syria or Iraq.
Furthermore, the distance between the two areas is daunting, being at least 2000km by land, or 1500km by sea just to reach Syria.
The second mechanism is actually quite likely.
The U.S. has provided assistance to at least three sets of groups in the area: the Iraqi government, Kurdish militias and anti-regime militias in Syria.
Due to the occupation of Iraq (2003-11) the U.S. has very good personalistic ties in Iraq and with Kurds, and consequently is unlikely to be misdelivering arms to those groups.
On the other hand, the Syrian situation is particularly murky.
It is likely that at some point arms were delivered to a Syrian group that made their way to ISIS.
The ultimate question, however, is whether or not that helped ISIS.
The answer is that it probably did not help materially primarily because ISIS' problem is not a shortage of small arms, which is what the U.S. is sending.
Since the fall of Mosul ISIS has had in its possession major weapons, including armored vehicles and helicopters, and the addition of machine guns or additional assault rifles is not likely to materially increase their capability relative to other groups.
politics.stackexchange.com/users/5309/The Pompitous of Love
I'm not familiar with the US helping IS in the manner you're suggesting, but it is common for opponents of intervention in the Middle East to argue that the US's continued intervention helps IS and other organizations recruit.
The argument is basically that terrorism is the natural response an occupier that can't be fought outright due to asymmetry; so, by this logic, the US's continued intervention in the Middle East inadvertently helps IS by motivating people to join them against the West.
And more generally, there are cases where the US has supported one side of a conflict and the arms given in support have ended being used against the US or its allies.
This is called blowback .
There are claims that some members of IS are former Syrian rebels that were trained/armed by the US, but I can't find a credible link that supports the claim.
(In other words, take it with a grain of salt.)
politics.stackexchange.com/users/4661/Tyler
Yes, the leading cause of terrorism is military occupation.
http://freakonomics.com/2015/02/13/is-there-a-better-way-to-fight-terrorism-a-new-freakonomics-radio-podcast/
When the US occupies Yemen, Afghanistan, and has bases in Saudi Arabia, Bahrain, Qatar, Iraq, and Kuwait, then that helps IS to recruit more terrorists.
politics.stackexchange.com/users/2430/Chloe
Also, I add, US and west aided the Syrian rebels against Asad regime.
Aid was in many ways, mostly weapons.
And we know, IS was one of the parties of the rebellion.
B Other than that, US disbanded the Iraqi Army during occupation.
Not only was Iraq drained of veterans and trained manpower, but some also say, these veterans joined the ranks of IS.
politics.stackexchange.com/users/5061/Rohit
Not only did the United States help ISIS , they supported them.
Who do you think was fighting Assad when they said he must go?
Once they made that statement they in-directly supported ISIS fighters.
politics.stackexchange.com/users/1452/Dammand Cherry
Yes,I include the entire developed in this not just the USA.
ISIS gets most of its funding from the direct or indirect sale of oil.
Every time we full up our car help increase the demand for oil.
We are not willing to stop trading with counties we know
allows their people to support the like of ISIS, due to wanting access to their oil.
It does not matter where we get OUR oil from, as by buying it, we stop it being sold to someone else, and therefore increase the demand for the oil
the ISIS and their supporters sell.
(Throughout history, it seems that the “locals” have never done well where there has been a natural resource in a country that could be exploited for great profit without the requirement to create great employment in the process.
The people in power WILL get rich, much better if they do so my creating employment for the rest...)
politics.stackexchange.com/users/4800/Ian Ringrose
Yes, there have certainly been incidences of the kind of accidental help you've been talking about.
The link below for example is to a story about how a US weapons drop via parachute was accidentally intercepted on the ground by ISIS, who took the weapons: https://www.youtube.com/watch?v=DsVaFpXgCfU Further, prior to those sorts of accidents when ISIS was first expanding into Iraq, ISIS fighters came across large amounts of weaponry that the US military left behind e.g. equipment for Iraqi Army soldiers that were left at the bases the Iraqis abandoned, during ISIS' surge into the North.
(Another report, on the above topic)
https://www.youtube.com/watch?v=m5Hn1GXBrG4 Cheers
politics.stackexchange.com/users/5381/TheRealPaulMcCartney
Occasionally it is heard from different sources that united states of America is accidentally helping ISIS through various approaches such as sending the parcels of bombs or other war weapons by airplanes and …? such as:     http://fa.alalam.ir  /  www.khabaronline.ir/World/middle-east  http://www.globalresearch.ca/u-s-general-we-helped-build-isis-islamic-state-obtained-weapons-from-u-s-consulate-in-benghazi-libya/5399141   The USA government claims they have sent them wrongly. Is there any data that shows that this shipments to ISIS are likely to be errors? (I.e. they make up a small percentage of all supply shipments)  Or does USA help ISIS in total? (in different ways)
Did the USA inadvertently help ISIS?
politics.stackexchange.com
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politics.stackexchange.com/users/4269/Shia_Sunni___________UNITY
The U.S. has probably not helped ISIS inadvertently, but arms have probably fallen into ISIS hands from the U.S. As the U.S. has provided assistance to the Iraqi government, Kurdish militias and anti-regime militias in Syria, occupies Yemen and Afghanistan, and has bases in Saudi Arabia, Bahrain, Qatar, Iraq, and Kuwait, then that helps IS to recruit more terrorists. While the US has probably not assisted ISIS directly, they may have done so inadvertently. Moreover, through US occupation of Yemen and Afghanistan, and US bases being present in SA, Bahrain, Quatar, Iraq and Kuwait, ISIS are better quipped to recruit terrorists. 
While the US has probably not assisted ISIS directly, they may have done so inadvertently. Through US occupation of Yemen and Afghanistan, and US bases being present in SA, Bahrain, Quatar, Iraq and Kuwait, ISIS are better quipped to recruit terrorists. 