put non-rechargeable batteries in a charger.  I know, I know: I should have known better.  But after a lifetime of playing it safe, I put a D cell in my charger.  About a half hour later it made a sound like a high-speed fan throwing a bearing and leaked steaming electrolyte all over my charger and sprayed material onto a wall three feet away!  I was fortunate that the spray just happened to hit the wall.  Had the spray hit me in the eyes I have little doubt that I would have been blinded.

I threw the battery out while it was still spewing its leaking innards.  Not much there to see except ooze.