It happened to both my brother and one of our hunting buddies on Friday.
My brother lined up on a nice sized doe around 3PM, 80 yards away, perfect broadside.
Ppppppop! - muted sound, watched a burning pellet go about 25 yards, saw his bullet hit the dirt about 15 yards from him. (Still in the sabot!)
Doe raised her tail and took a few hops, then stood at about 90 yards looking around, then back to grazing as he reloaded.
2nd shot was true and dropped her in her tracks. Turns out he had loaded with the last 2 pellets from a 5-6 year old box.
Speed loader tubes had fresh pellets.
Our buddy had the same thing happen, except he didn't get a second chance at the shot. His doe was only 50 yards away and high tailed it outta there. Same deal, old pellets.
I shoot mine a bit year round, so I don't have pellets for more than about 9 months, a year tops... but I still vacuum seal it between trips for cheap peace of mind.
I seal up my primers as well.. can't hurt.