I stumbled across this m4carbine.net thread about the “craziest failure I’ve ever seen”. It discusses a situation in which a round gets wedged between the upper receiver and gas tube of an AR-15. I realized quickly that I’d seen the malfunction before but never known the name of it. I also realized that the clearance drill was never covered in my Front Sight 4-day Practical Rifle Course.
The only time I’ve ever seen a “brass over bolt” malfunction was at a range in Arizona many years ago. I was borrowing an AR-15, using a used surplus 30-round USGI magazine, and shooting with the weight of the rifle supported by the magazine. At the time, I assumed the magazine was the culprit, so I tossed it in the trash and moved on. But clearing the malfunction took me another 5 minutes and a but of headache.
I found a couple YouTube videos that cover the clearance process: