Yes, the word is that for some LED lights reversing both batteries will fry the LED.

One time, when travelling, I cut a circle of white plastic from a yogurt container lid and put it between the endcap spring and battery (in one of those AA Minimag-style lights; in this case, a Fenix L2T). Perfect electrical isolation, and I could put the light back into action in less than ten seconds, even in the dark.