I recently got pissed at my old Yamaha PSR-330 keyboard, that kept freezing up within a few seconds after being switched on. I grabbed a screwdriver and got to work.
Suspecting a cold solder somewhere, I got around to finally open it – and, to my surprise, my searching glance found a weird blob of something that looked like spilled battery acid, though no batteries were ever near the cartridge slot where the strange growth lay. I swabbed it up with some alcohol, but then I noticed a worryingly loose pin on one of the chips near there. It almost lacked any solder and could probably come completely loose when pushed a little, so I painstakingly soldered it… to a pin belonging to a resistor, that was conveniently sticking out nearby. A shabby soldering job if I ever saw one, but hey, it’s the Yamaha guys that started it!
Anyway, the keyboard has now been working for several hours without failure.
Update: was working, past tense. Worked and then froze and never rose again. Bugger.