I accidentally reverse-polarized mine the other day. That night I discovered the light's battery holder body melted/distorted. Unable to remove batteries due to distortion. The heat generated even left a faint dimple in my SKS fender that the light rested upon.
I had to saw it in half and beat it apart with hammer for the autopsy.
Batteries installed backwards. No other apparent clues to failure.
I had done this before but had discovered it right away as I was heading out for ride and had to turn light on. In this final case, I installed batteries (backwards) and came back to ride hours later, damage already done by then.
IMO these things should have RP protection in the control circuit. Can't cost much, and it's not that hard to throw batteries into these holders backwards.