When I visit Stack Overflow and I've got the orange notification bar at the top of the page, there's a little "X" to close the bar. However, that X is not centered vertically within its box. This has bugged me for a long time and I figured it was a temporary thing. But it's never been fixed. Now I'm chalking it up to sloppiness.
As seen in Safari on Lion:
I've deleted Arial from my system (it's ugly).