I left my inflated Thermarest NeoAir Regular in my tent (Tarptent Contrail) for a short dayhike. When I came back, the heat (up to 100 degrees) from the sun on the tent had expanded the air in the mattress and popped 4 or 5 baffles, causing a basketball-sized goiter-like inflation in the middle of my mattress.
Wouldn't have been so bad if it had been on the end of the mattress (a pillow! or footrest!) or not on the very 1st day of 4 days out!
So – If you're gonna leave your NeoAir in your tent in the sun, open the valve and let a bunch of air out first!
Never had a problem with thicker ThermaRests, but I've probably just been lucky..

