I've seen this type of jam happen in both 870's and 500's. It's most commonly complained about in newer 870's where the chambers may not be as highly polished as older versions and they're not broken in from use. Oddly enough I've seen it most often in Older 500's and 835's that have had literally thousands of rounds through them. Never had one of my 500's or my 835 do it but several of my hunting group have in my presence. If it's a spent shell and you're in the field, hold the pump and whack the buttstock downward on a somewhat yielding surface (cornfield works great
) and the shell will either come out or the extractor will tear the rim off. If the latter happens use a cleaning rod to knock the shell out of the chamber and go back to work (until it happens again).
In the 500's and 835's I've seen it happen with it's been an intermittent problem and it's happened with more than one type of ammo, usually 3" or 3.5" 00 buckshot of one flavor or another. Never seen it happen with birdshot.
In 870's I've had it happen with one particular barrel that hated Brenneke KO slugs for some reason, but never with any other barrel (out of a few dozen that have come through my hands over the years) on any other gun. It's been widely posted on the internet to be a common problem to newer 870 expresses, particularly when combined with cheap value pack birdshot loads.
It goes without saying that if you end up with a LIVE round in the chamber, disregard everything I said in the preceding paragraphs.