Icon change

Every so often icons for a certain type of file will change, seemingly by itself, uniformly system wide. The program that opens it stays the same. After a reboot it returns to normal. It doesn't cause any problems that I can notice, but any one know why this happens?

probably a corrupt icon cache.  it automatically fixes itself on reboot, and doesn't do anything worse than show the wrong icons.

if you want to fix it manually, delete C:\WINDOWS\ShellIconCache (a hidden file).