They're/There/Their: Okay, yeah. This one just bothers me. If one is using, "they're," it can be replaced with, "they are." If one is using, "there," it is a place. "Their" is a possessive.
"They're going to put their things over there."
"They're from Kentucky."
"Their gifts arrived just last night."
"The gifts are over there."
Usually, the difference it clear. However, it is not always clear by the time the reader has reached the word, meaning s/he has to reread the sentence, and that's no fun!