Not in the boot , but I did lock the key in the ignition one of my W124's when I was changing the battery , and the central locking locked itself !
I had just bought the car , which came with only the one key , spent ages sucking and blowing at vacuum lines to no avail , pulling and replacing fuses , ditto . In the end I broke a rear quarterlight , then got Autoglass to come out and replace it for the £50 excess - expensive lesson . I had intended getting another key , nowadays a spare key is one of the FIRST things I get if a car comes with only the one .