The cap can unscrew in your pocket, or tighten up and inadvertantly turn on (not good for battery life).
When I tried to use it once, the light was on when I pulled it out of my pocket. The next time it didn't work, I thought the battery died, so I replaced it--thinking it was on in my pocket forever.
It's a new light, so I sure hope nothing has had time to oxidize, but I think I'll give it a check and maybe a grease treatment anyway.
What's your favorite ARC?