Following from the Porter Airlines off-topic discussion, air your spelling, grammar, and communication pet peeves here. We haven't had this discussion for a little while...
I've noticed that while the level of discussion here is far above the average for internet forums, there are still a few common mistakes - even from some of the better writers.
1) The one I keep noticing recently - sometimes it seems to come up in almost every thread - is the mistaken use of an apostrophe in the possessive pronoun "its". (For many people, this is not a confusion with the contraction of "it is" = "it's", but a confusion with the possessive apostrophe.) While English uses an apostrophe to mark possession (e.g., the dog's tail), possessive pronouns (ours, yours, theirs, hers, its) do not use an apostrophe.
The dog's tail was wagging.
The dog wagged its tail.