I have had similar issues since upgrading to Mountain Lion. It has only affected Mail, and has only happened a couple of times. In both instances, quitting mail and restarting it has fixed the issue. I didn't think anything of it, and assumed it to be a bug in the new version of Mail
Assuming that nothing has been blurred out by Photoshop, that looks like a graphical bug. If it is the first time you've seen it, it could be nothing. However, if it starts appears more often you could have a defective GPU.
There's a good chance it's not a defective GPU.
OP, are you running Mountain Lion? Ever since both myself and my friend updated from Lion, we have been getting these graphical bugs in Mail. If these odd glitches occur elsewhere, then you might actually have a problem. Otherwise, just wait for 10.8.1 and see if your issues are solved.