I was wondering that myself..I've finally decided to NEVER buy another glass one again.. I bought a new 25 watt marineland glass heater a little over a year ago. It has a lifetime warranty on it.. I had it heating the water in a trash barrel for a water change and I notice the house lights dimming for a second.. Hmm whats that.. then again and I can smell the familiar burnt smell of electrical stuff burning.. UHOH, its the lights on the tank i think.. i finally figure out its the junk glass heater has broken the glass and dropped 1/2 of it back into the water, ruining water(luckily only that). It's been running submerged for several days, no banging against anything, no fish to cause breakage, no changes from dry and then submerged. No other casues except it's junk! It's in the trash right now and sending it in for replacement does not appeal to me.
BTW, no I dont gave the GFCI installed on that outlet, but will very shortly!