This isn't exactly a glitch, but it was a major WTF on my part, so here goes.
I had a friend over, and I wanted to show him Fallout 3. He was dicking about in the wasteland when he was attacked by a pair of lone Super Mutants. He felt like having some fun, so he whipped out a missile launcher with around 30 rounds, and VATSed them. And missed both shots. So he tried again. and missed once more. This went on until all 30 of the missiles were spent, and in the end, he hadn't landed a single hit.
Seriously, those Super Mutants must have either had the best luck in the world, or my friend must have the worst...