I have a mechanical room with an outside steel door in which the plumber had a little accident, so I set up a parabolic heater in order to dry things out.  This went on for two weeks until one evening I noticed that the painters had neglected to replace the door seals, so I put them in before I went home.  Next morning I was greeted with a strange buzzing sound.  With the seals back in, the room heated up so much that the parabolic heater sensed an overtemp and shut down with an alarm.  I thought it was pretty amazing that much heat was being lost around the edges despite having a good fit.

It was a good lesson on just how much heat you can lose through small cracks.