That happened to me once. My line was still on the bolt snap, too. A sharp edge in the light's lanyard hole had cut through it. Now, instead of tying on cave line, I use 2 small zip ties to attach my bolt snap to my light. Haven't lost a light since.
Oh, and on the 1600's "lock": I've had mine locked and then turn on accidentally - during a dive! - more than once. It appears that it just gets pressed between my arm and body or something. And, of course, it's clipped to my shoulder strap and pointing more or less backwards, so I don't even know it's on until a buddy tells me.
My current protocol is to carry a 1600 as my primary - i.e. in a soft Goodman on my hand - and have an 1100 (or 2) locked and clipped to me as a backup.
Are you worried about the zip ties might "crack" when you least expect it? I know after some time they get brittle. What's your thoughts on zip ties vs cave line with super glue?