Empty round stovepipe. Usually noticed on the last round, but can occur anywhere.
Too much recoil spring. Weak or broken grip. Low extractor tension. Extractor clocking. The clocking extractor usually results in the last round being crunched by the slide, even though all others kick out of the port well. The extractor still has a grip, but the rotated hook drops it due to not having a convex surface to support the case. The magazine follower is depressed far enough to prevent slidelock, and the case rides the slide back toward battery. Crunch. I've seen the empty brass actually get stuffed back into the magazine on occasion.
http://www.thehighroad.org/showthread.php?t=61238