It was "kittens coats". If one of them took another's coat, he should give it back.

Apostrophes may be used in plurals of numbers (as above) or in plurals of acronyms. "The DVD's are over there." Personally, I don't use them in plurals unless there's a problem with readability because they look silly.

I got one wrong, and I got it wrong because I thought, "Hmmm. I'd stick a comma there, but I'm probably wrong, and there IS no comma."