When the gun cycled the last bullet in the magazine, it jammed a live round in the chamber and the slide was partially closed shut. After some very careful persuasion I got the slide open and here's what came out of the ejector port (below). The only way I can see that happening is from it feeding.
No problems before, and no problems afterwards. Cycled a few rounds in and out of the weapon before firing it again. Gave the gun a thorough cleaning afterwards - didn't see any problems.
Is it possible I just ended up with a bad round that was slightly deformed?