Common Theme Problem: "Write Error on file _themes\"

When placing custom SharePoint themes into the 12 folder, sometimes you may run into this error when you try to select your new theme:

“Write Error on file _themes\

Typically, this error happens when you are making changes to a theme and decide to make backups of some of your images by doing a simple file copy and paste. I’m not exactly sure what about this process causes the error, but simply moving the copied files to the desktop (or somewhere other then the theme folder) should correct the problem. Be sure to perform another IISRESET before re-selecting the theme.