Searched the forum, but didn't see anything quite like what happened to me, so I finally registered to share my disaster.
I bought a 50W Petco heater
about a month ago, to heat a 2.5g experimental/holding tank. Figured it was cheap ($17), could maintain a chosen temp, and there was little chance of overstressing the heater. Ran pretty solid for a month, kept temps between 78-79 all the time. A few days ago it decided to be a little less reliable, didn't heat quite as well, slowly lowering the temperature it could maintain by about a degree per day, but still well above room temperature. I kept it plugged in since it was still semi-functional and as long as it wasn't always on, no chance of a fish fry or explosion.
Well, yesterday afternoon, I decided I was going to go invoke the petco warranty since it wasn't working anymore. Verified that the temperature of the tank was indeed crap - down to 72 degrees with the dial turned all the way up, then started looking for my receipt.
As soon as I turned around, BAM! Sounded like a gunshot in my bedroom. I looked around to find the cause of the sound, then immediately noticed water glass and gravel all over the floor. I looked up to see that my tank had shattered! The entire back of the tank had blown out! My fish managed to get stuck either on the gravel or on top of the bookshelf, so I threw them into my 10g (which is now probably lightly overstocked), and started an hour and a half long process of cleaning up shards of glass, gravel, and heater bits from my wooden floor and carpet. Luckily petco refunded the heater under warranty, took back the other one I had on my 10g out of pity, and so I used the money to get a 50w visitherm stealth.
Attached pictures of the tank for your viewing pleasure. You can see the Red Sea Nano/Azoo Palm filter still sitting on the side. The explosion managed to crack the intake as well, but some superglue mostly took care of it.