cadman:
The incorrect interchangeability of "then" and "than", "you're" and "your" and "been" and "being" from people that are otherwise apparently intelligent. It makes reading their sentences twice as long as you have to go back over it again substituting in the correct word once you get to the point it no longer makes any sense.
Let me add to this... "bought" and "brought", "used (to)" and "use (to)".