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.