I have had this problem twice with my 12 gauge mossberg 500 and a friend of mine has had the same problem.
In all cases the culprit is a small piece of underbrush, a seed or something else caught in the trigger assembly. Usually, dissasembling the slide and dropping the trigger assembly out will take care of it. Be careful not to lose the small BB that is under the tang slide itself!
One time it required the services of a gunsmith to take the entire trigger assembly apart. He said there was quite a bit of brush in there!!
I had a bad habit of holding the gun in front and away from me while busting thick cover. My buddy never did that but he still got a piece of milo in there somehow and it jammed up the tang safety.
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