28 gague hulls ....( especially Remington STS ) have been breaking the plastic hull on the inside of the shot cup...and they will sometimes, end up going quite a ways up into the barrel.....and its been happening on some new shells, on one reload, on 5 reloads, etc....../ it was a really hot topic about 3 - 5 yrs ago.
But that problem - at least with Rem STS has neven been an issue on 12ga, 20ga or .410....
Remington is aware of the problem - says they've changed the source on the extrusion they're using...but who knows.
My hunch is its a combination of issues - quality of the plastic extrusion and how it is crimped or glued or whatever they do to seat it into the brass cup. ( they might use heat - and just jam them together ) ....but when they break there is still plastic left inside the shot cup.
My way of dealing with the 28ga ....when I fire the shell....I twist the plastic hull inside the shot cup in my hands after I remove them from the gun ...and before I drop them in my vest. If they're solid - I reload them / if they move at all or feel a little loose - then I dump them.
When it happens - its easy to fix - put a cleaning rod down the barrel and tap the broken hull out. I have seen the spent shells fracture ...and the bird break normally ...so you would have no indication of an obstructed barrel in a pump or semi-auto ( so if you're going to shoot 28ga --- its a good reason to stay with an O/U )...because when you pull the shot cup out in a small piece...you know the rest of it is in the barrel.
I have seen it happen on 28ga Estate as well....but not as commonly as on Remington STS. ( I had another STS 28ga break last week on me - shooting skeet).
But the OP was talking about 12ga and 20ga - where this is not an issue.