I ran into this video through the @AdviceToWriters twitter account and I thought it was quite funny:
Deciding when to use an apostrophe is hard for learners of English as well as for native speakers. The most problematic one is definitely <'s>.
So just to make it clear, these are the three situations where <'s> is grammatically correct:
1) When it indicates possession: This is Christine's backpack.
2) When it substitutes the i in IS: Kevin's happy.
3) When it substitutes the ha in HAS: She's got a nice cat.
That's IT!! So, NEVER use <'s> for plurals!!! (you'll find plenty of those mistakes in the video)