Color-less Classic -- A friend at Apple notes that Color Classic users can move the contrast slider bar in the Screen Control Panel all the way to the left, making the screen go pitch black. It would seem that users in that situation are stuck, since they can't see the slider bar any more, but pushing the screen contrast button on the Color Classic's front bezel will bring the screen back up.
Mac OS X Zip Expanding Utility
Firefox (and possibly other applications) may ask you what you want to do with .zip archives that you download from the Internet. If you want to expand them with Mac OS X (rather than StuffIt Expander), you may be unsure of which application actually does the job. You're looking for Archive Utility (in Leopard and later) or BOMArchiveHelper (in Tiger). In either case, the application is stored in Hard Drive/System/Library/Core Services/. Don't move it from there, though, or you'll confuse matters.