Today I was playing zombies on the theatre map when we got a firesale. The current mystery box location was in the dressing room. I used the firesale mystery box in the Alley and got a china lake (not a good gun in my opinion for zombies), but the box took my gun when it just appeared and disappeared with the girl laughing in the background.
I played through the next wave of zombies and went back into the Alley and saw the mystery box there. The green light for the mystery box was still lit up in the Dressing Room. I thought the lights were messed up so I went and checked it. The mystery box was there also. So I got one of the other players to be in the Alley at the same time I was in the Dressing Room, and sure enough there were two fully operational mystery boxes that both cost 950 points.
I'm not complaining about this glitch and I was honestly too lazy to get a camera to record proof of it occuring. Also I'm not sure if it works on any other zombie map or if it was just a one time thing.