Scrollbars and Firefox on OS X
Scrollbars bleed through absolutely positioned elements on Firefox Mac 2.0.x. This is similar to how <select> elements bleed through in IE <= 6. The difference being that there's a fix for the latter.
There was a moment of hope when I noticed that setting the positioned element to overflow: auto.
However, they bleed through again when the window loses focus.
Turns out the bug is fixed in the repository head for version 3, but they're not planning on rolling a fix for 2.0.x. This makes me sad.