A few years ago I was walking down the street when the metal lens cap off my Elmar 50 popped off and right before my eyes, rolled down the street and into a drain. I was silently stunned for a minute and decided to try and forget about it. A few days later realizing that these embossed Leica caps were about $20 a piece I went back to the drain, found the cap, moved the grate, pulled up my sleeve and rescued it.
I am now paranoid about lens caps, and use boring, cheap plastic ones whenever possible.