Many years back when at a campout with my son's Boy Scout Troop there was a parent visiting that was wearing some kind of water bladder backpack. She plopped down in a chair and then quickly sprang back up.

It turned out that she'd put ice in her bladder, and the bladder burst open when she sat down - flooding her back and rear end with ice water.

I've always been under the assumption that it was a cheap imitation and that the good brands would not have failed so violently (so to speak).