Don't shoot your wad

A Mossberg 500 Kaboom with picture. The actual reason for the Kaboom is not known for certain, but the consensus is that a 20 gauge shell was chambered first, followed by a misfire and the racking of a 12 gauge shell. There is also the speculation that a plastic wad got stuck in the barrel and the shot from the next load hit it, causing the Kaboom. Who knows?