The weapons were intended for Syrian Kurdish fighters.
Small arms, ammunition and other weaponry were dropped in 28 bundles on Monday by the US to help those defending the town of Kobane from IS.
A US spokesman said 26 of the dropped equipment ended up in the right hands.
The Pentagon says one bundle of weapons was taken by IS fighters while a second stray bundle was destroyed in an air strike.
So how did it happen? And could the same mistake be made again?
Dr David Betz from the Department of War Studies at King's College London is an expert in strategy, insurgency and counter-insurgency.
Newsbeat put these questions to him.
Air dropping supplies to forces on the ground is never a first choice.
It's costly relative to other means, and though it is a well-established and a long-practised technique, it is also relatively inaccurate, for obvious reasons.
It's done when there is no other alternative, most often as in this case, because the supported force is effectively beleaguered.
It would seem that Turkey is not permitting supplies to the Kurds fighting in Syria to cross its border.
If it were it would be much simpler, more efficient, and effective to do it that way.
Often. That is why it's a last resort. Military history is full of instances of surrounded forces being supplied from the air.
Think of the Germans at Stalingrad surrounded by the Russians in the winter of 1942-43, the British paras at Arnhem in the Netherlands surrounded by German forces in 1944, American marines at Khe Sanh surrounded by North Vietnamese forces in 1968.
Or in a slightly different way the Allied supply of the French Resistance throughout the war.
Sometimes it works - often it doesn't.
As noted above, it is a well established skill and no doubt armed forces have gotten a lot more accurate over time but there are inherent limitations of accuracy when dropping things on pallets dangling from parachutes into an active combat zone where forces are intermixed in close proximity.
Undoubtedly they are deriving propagandic value from it, as the article suggests, but that is not to say they are making it up.
I would say all of the above, of course. Weather, enemy action, pilot or navigational error, equipment failure...
War is inherently chancy and it makes the simplest things difficult. I wouldn't necessarily characterise the drop as not going as planned.
The planners would have known quite well that there was a strong possibility of some items going astray.
In fact, they'd know to a good deal of precision what the per cent would be.
It goes back to the previous point of having no better option.
If the only way of getting however many tonnes of military supplies to the Kurdish forces was to drop them in a way that IS got its hands on a fraction of them it might still be judged worthwhile to do so.
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An Islamic State video which shows militants with US weapons is being investigated by defence experts at the Pentagon, the US defence headquarters.