The one that ticks me off the most is the following depressingly frequent display of etymological ignorance. just for the record and so that there is now NO good reason to cock this up ever again....
There - as in descriptive of a particular place (physical or temporal) as in "over there" or "Once upon a time there was a happy little spark called Carter..."
Their - as in something belonging to them
They're - as in the contraction of the phrase "they are"
In summary and for instance, if someone were to ask you "where are the pipe-fitters today?" your answer would of course be "They're over there ----ing it off in their portakabin"
Phew glad I've got that off me chest.