bulgie 

Yes it's conceivable, but in all the 60+ years those have been in use (Universal 61 being possibly the first), I have never heard of it happening. (OK I'll admit I could have heard about it and then forgot, but that also seems somewhat unlikely, it's the kind of thing I would remember.)

Plus, I trust that I would feel it if it happened, and click it back. The scenario where I knocked it open without noticing is just too vanishingly unlikely to bother worrying about, for me. Maybe I'd worry about it for a child, or an adult who's just very much not mechanically inclined. NTTAWWT! Some of those un-mechanical people have skills I can only dream of!

A co-worker at the bikeshop once accepted a ride home from the bar after being over-served. To put the bike in the car, the kindly designated driver (another bikeshop dude) disconnected the canti brake straddle cables and took the wheels out, then put the wheels back in at the drunk guy's house but didn't hook the brakes back up. Next morning, my hung-over friend jumped on his bike for his commute to work, which starts with a long downhill. It wasn't until it was time to brake that he noticed he didn't have any. He crashed through a hedge, with some tearing of flesh and significant damage to the hedge, then went across a parking lot still way too fast and crashed through the hedge on the other side. No stitches required, but a lot of bandaids! Moral of the story, don't let anyone else work on your bike! OK maybe that's not the #1 moral, something about demon alcohol maybe? Or, "when you have no brakes, look for something soft to crash into or through."