Artifact on Firefox 4 on a Mac

Mac and Firefox
Recently we had a odd bug pop up on one of our projects. It only showed up on Firefox (3.6 & 4) on OS X. The issue was that on input fields (in our case a radio buttons and text entry fields) there was an odd graphical artifact that showed up far to the right of the entry field.

Mac artifact

It took some digging and a lot of experimenting, but we finally tracked it down to a line of CSS:
overflow: auto;

When I changed that to overflow: hidden; the problem went away.

I didn’t bother to dig any deeper into the problem or try and figure out the cause of the problem yet…I will when things settle down.