I got burned today by an (apparently) undocumented change to the way that ColdFusion finds Custom Tags. This took a little bit of stomach acid, and a lot of patience to figure out.
It appears that when ColdFusion 7 tries to invoke a custom tag from the CustomTags folder, it look for the tag in sub-directories to the CustomTags folder before bothering to look in the CustomTags folder itself. So, if you have an old copy of a custom tag in CustomTags\old_tags, and a new copy in CustomTags, it will call the one in old_tags, and ignore the newer one. This is very counter intuitive, and was not the case in older versions of ColdFusion, including CF 5, and CF 6.1.
Furthermore, if you delete files in the sub-directory, ColdFusion doesn't realize it until you restart the server. It just gives an error.