Picked up some brass laying around at the range yesterday and didn't pay much attention to get until I got home and sorted through it.
A couple of real odd straight cases turned out to be .308 rounds that had apparently been fired in a .300 magnum. There was no neck left at all and the cases were severely expanded out to magnum case dimensions. One case was missing the primer and the other indicated about normal pressure for a .308. Fortunately the brass just expanded and didn't split and dump a lot of gas in the shooters face.
I've picked up quite a few mis-matches over the years but this has to be one of the most dangerous! I guess it pays to look around you and see what the guy on the next bench over is shooting.