I also have had heaters malfunction and fail on. I lost a tank full of angelfish to them.. I also lost another tank with a mix of fish to one. Over the 60 years I've kept fish , I don't think I've had more than a half dozen heaters fail on .. I don't recall ever having failures in summer though. When they fail on, it is always full on.. Heaters are always at their rated output when on .. They regulate heat by turning on and off as necessary to maintain the set point. Way back when, a lot of heaters and thermostats used to be separate, and virtually all were serviceable. Now they are a sealed unit that either works or fails, and yes, most failures are in the on part of the cycle...It is seldom the heating element burns out so the heater will not heat. The only thing I take exception to is the idea that unplugging the heater in summer will somehow keep a tank cooler. It doesn't. A properly functioning heater should never come on if the tank is above its set point. A heater can't fail on unless it is already on .. What happens is over time, the points in a heaters thermostat arc slightly every time they make or break. That causes them to pit and build up with metal from the pitting. Ones that have had that crud build up too long and too much can actually fuse closed keeping the heater on.