While working on a site I'm developing, I noticed a strange problem where the text in a certain paragraph bolded when the page scrolled (see right).  The bolding was clearly a rendering error, since the bold did not extend to the end of lines and would disappear temporarily when another application opened or saved a file (only to reappear when the page was scrolled in the browser again).

My Setup: Firefox 7.0.1 on Ubuntu 11:10 (32 bit) :: Mozilla/5.0 (X11; Linux i686; rv:7.0.1) Gecko/20100101 Firefox/7.0.1

The Solution:  In the limited time I had, I wasn't able to find a software solution or a bug report on Bugzilla, but I did find a website fix that avoids the bug here.  Add "opacity: .99" to the body element in your CSS file.  That should fix the problem for everyone if you control the site, but isn't much help for those you don't.