Thought I'd link this here; if the mods would like to move it...
The readers' digest version is that a BatteryJunction B65-clone for a Maxfire had a positive terminal almost wide enough to touch the inner wall of the light, and it was held in place by welded nickel(?) strips - with enough play to allow a short. I missed the sloppiness at first.
Moral of the story is to understand enough battery-pack engineering to spot design flaws.