I have ruined two heaters to this. First time it was me draining my 75g without unplugging it first, it was one of those black plastic Marineland ones that were recalled awhile back. It started to expand then smoke. Second time I had one with cord wrapped around it and in a box under my tank. Went to plug in my circ pump after a change and mistakenly plugged in that heater, cooked melted the cord around the heater.