A is for …
Apostrophes. Misplaced apostrophes are every pedant's favourite bugbear. Whether they're missing or superfluous, there's plenty of opportunity for getting it wrong. Apostrophes signal a missing letter (isn't = is not) or possession (Andy's jacket = the jacket belonging to Andy). Remember, plurals never take an apostrophe. I wrote about the really common confusion between its and it's to illustrate the point.