I just plugged my MD-UWII+ power supply into my A4 without noticing, and got no sounds. Checked the volume, reset the sound, nothing. Checked the headphones, and the jack converter, nothing. Then I realised that I had the wrong power supply, and my heart jumped straight out of my throat. I plugged the right one in, and the A4 started acting all weird, not resetting sounds properly, having effects on sounds which I know I never added. I kept trying to restart it, reset and change banks, etc., but eventually, simply playing the two patterns I have in there at the moment seems to have reset it and it is working again.
Knock on wood. I already have stickers on the power supply itself, but I am going to add ones at the end now too. May I never do this again, that was too scary!