I was just doing a little maintenance on my CPR overflow... and broke the bulkhead! I must have tightened it too much and the threads separated from the flat inner portion that's on the wet side. My first reaction was "Fahk! I just ruined my overflow and broke the connection from DT to sump/fuge. I hope the corals and fish can hang on until I can get a replacement at the LFS tomorrow". Then I started thinking "what do I have that I could use to rebuild the bulkhead?" After rummaging around in my totes of miscellaneous aquarium gear, I discovered that the screw-top for a small nalgaen bottle fit the threads of the bulkhead. So, I drilled the center of a Nalgean top using a 1" paddle bit, grabbed a rubber washer I'd saved from some home improvement project, and suddenly I was back in business with a watertight seal.
Moral of the story... its handy to have random aquarium stuff available so you can MacGyver/MacGruber a temporary fix.