I guess I owe a disaster after starting all of this.
This was years ago with my old Pentax K1000 and I was in HI and up on a ridge about 10 feet from a cliff. I had been trying to take some bird pictures but decided to change to a 28mm and go for scenery (some mist was starting to rise in the valley and the sun was sinking).
Sit down, get out 28mm, loosen the cap on the base of the lens, unlock zoom and remove it from camera, dump cap in my lap and set 28mm on camera, put cap on zoom lens, put zoom away, stand up, and watch 28mm drop off camera, land on it's side, and start rolling towards the cliff. I had forgotten to lock the lens to the body. My kid sister grabbed the lens and, after examining it, I realised that I had been lucky and there wasn't even much dust on the optics.
I was in High School at the time and, needless to say, I decided that I should probably work out a better method for changing lenses.