Question about the failure: would the spent shell stick with the bolt fully closed or would it extract partially, then stick?
It wouldn't open at all. I'd fire and try to cycle the gun, but the handle wouldn't budge at all.
On anther note, I'm very amused at the horror and awe of my 870 not being detail cleaned in years. Folks, it's a field gun, that I got in High School 20 years ago, explicitly to bang around in the field and generally treat like crap. A gun that I can literally drop in salt water and not worry too much about.
Sure I probably "should" clean it more often, but until now I've never had a problem.
Certainly my Drill Sergeant from Sand Hill many, many moons ago would lose his mind if he were to see the weapon like that, while unleashing his PhD in creatively combining 4-letter words and I'd be doing "Little Man in the Woods" all night long, but fortunately that's no longer the case.
Some guns were meant to be babied and I certainly do baby them, cleaning them, talking to them and caressing them (my Skeet O/U, my S&W Model 17 and other high-end guns), but this isn't one of those.