“FREEFALLE7”, a Ranger Instructor, posted the above photo at AR15.com. A student, on arrival at the Swamp phase of Ranger School, showed him his M4 and said “my barrel fell off”. His punishment for not telling an instructor at the Mountain phase (the previous phase) about the broken M4 was to be given a M240, which weights 27 lbs, as a replacement.

Keith J came up with a plausible theory of how this happened

Salt water in the threads. Steel rusts. Steel in contact with aluminum. Aluminum then oxidizes, reducing the rust to iron dust. Joint remains tight until it is abused, then it just falls apart.

This all started when the barrel was installed and it was tightened a bit too much, causing the anodizing to crack

Of course this would have happened over a long period of time and use.

Thanks to Jay for the link.