A common one in chain pet stores is zoomed. They're made to be usable for many different species so the temps can get really hot. The thermostat I've got attached to my ball python's came unstuck while I was cleaning yesterday and it took maybe an hour for it to get to 96 degrees (luckily I caught it before it went any higher). But that brand can get hot enough to burn a python, so it's definitely capable of burning a frog.
Another problem is that the internal wires can be damaged by bending or something like that.