A lot of people write NPC's when they actually mean NPCs, clearly theres something very wrong going on, but why ?
For example if they want to say that NPCs should have birthday parties they write it this way: "NPC's should have birthday parties", which has no meaning really.
The 's is a possessive pronoun, while just "s" is used to form a plural of the noun.

http://grammar.about.com/od/punctuationandmechanics/tp/GuideApostrophe.htm

Please advise because getting this wrong makes your posts look not as serious.

We don't play at grammar cops here, there are lots of different nationalities posting on these boards and not all have the full understanding of English grammar. If you can understand a post, fine, if you can't then don't post, complaining is a bad idea and makes your posts look not as serious.

This has nothing to do with FNV so I am closing it.

You are correct about the apostrophe use, although you chose an odd subject line for your point. However I see no benefit or reason to have a thread on this subject here.