If I hadn't seen it with my own eyes...
Epoxy glue used to seal an ASA leak.
Toothpaste used to clean a marker (and give it that fresh, minty feeling).
Scotch tape wrapped around the back to hold the grip frame to the gun body (to replace a missing frame screw).
JB Weld gunked around a steel braided line to stop a leak.
Rubber band o'rings. Literally. Rubber bands!
Holes drilled through goggle lenses for ventilation.