I just threw together an article on how to fix an annoying MMC error that I've run into a bunch of times. I ran into it again this morning so I figured I'd share the fix for the following error:

MMC cannot open the file C:\WINDOWS\SYSTEM32\GPMC.MSC.

This may be because the file does not exist, is not an MMC console, or was created by a later version of MMC. This may also be because you do not have sufficient access rights to the file.