What might cause a a dock icon to have pixellated noise? This is possibly a non-issue, but the Finder icon that appears when you Cmd + Tab has some strange graphical noise on the top. I can't remember how long ago it happened, or if I made anything strange before it appeared.

Does anybody have an idea of why this happens? I'm on a 13' MacBook Air 2011 with 10.7.5.
It's likely the Finder's application icon is corrupted.

To check, navigate to /System/Library/CoreServices/ and select Finder.app.  Get information on the file and see what the icon looks like.

I'm not sure where you can get a replacement icon (maybe another computer or the install disks with some package exploration).  I'd post mine here but for Apple's remarkably strict copyright lawyers.

--SOLUTION--

Navigate to ~/Library/Preferences and move the com.apple.dock.plist file to your desktop. This may result in your dock items needing to be reloaded. Logout and back in.