Gear knobby thing
 

Sorry don't know it's right name, it's basically a bolt with rubber on it sitting at the end of the lever for you to flick with your toe. Riding along yesterday when as I changed up I felt it pop off. Luckily I was able to find it & screw it back on, may not be so lucky next time. It just seems to be a screw fitting that's worked loose.

Is this a common problem & should there be a bolt at the other end of it to stop it doing this?

Re: Gear knobby thing
 

Thread lock is your friend.