Here's a small bug I've just faced: if you display someone's reputation, and you click on the small triangle to display the reputation, there's an AJAX call that is made.
Usually the answer is so fast that there's no problem.
But when the answer takes too much time, you have the time to click once again on the triangle, and to re-click for the third time (follow me?) so that there's another AJAX call that is made.
The result is that when both AJAX answers come, they're both appended, see here: