I've never had this happen, but I'm always super paranoid about it happening. I only have one rifle on the bench at a time and one box of ammo. Then, I double check the headstamp to the caliber stamp on the barrel.

Wow, that's bad. Maybe a 30-06 in a .270? Not sure if that would chamber or not. Closest I have come is firing a .223 round through a 6.8SPC AR-15. It fired ok, but the case got stuck in the chamber until I could get home and drive it out. It didn't damage the rifle. I keep my magazines for 6.8 marked, but my wife got them mixed up. I'm always diligent at the range since I usually have several rifles I'm shooting, but will always move the ammo off of the bench when I switch between whatever I have out.

Update: It wasn't A Magnum case after all. 308 Winchester in a 25-06.

That would do it. I wonder how far that 308 slug made it down the barrel before she gave out.

I watched a 41 mag get shot out unintentionally of a 45 LC Marlin one day. Sounded real funny, split the case, but he hit the limb we were shooting at.

Had a customer bring my stepdad a Marlin lever action in 357 mag that they had loaded 357sig in and jammed up the action. He disassembled the gun and when he was dumping the ammunition out of the magazine one of the cartridges detonated in the magazine. He had his open hand under the magazine to catch the cartridges as they came out and wound up losing his middle finger over it. The gun was easily repaired but his finger was not.

I had a new out of the box Colt Sportster slam fire out of battery when I release the bolt catch. The receiver was cracked, the magazine floor plate blew out, and I got a nasty slice on my thumb somehow. I sent the gun and some sample ammo back to Colt and they gave me a brand new gun that somehow had the exact same serial number and a ton of swag to go with it.

Very similar circumstance to what I saw happen at the gun shop / indoor rifle range I worked at back in the '70s. Shooter was checking two rifles before a Colorado hunt and inadvertently dropped a .308 round into his Sako .264 WM. Results weren't as catastrophic as this but were certainly impressive.

The slug was swaged down and only lacked about 2" of reaching the muzzle. The case expanded to failure and released the still-expanding gases rearward through the action. Most dumped into the magazine well where the box expanded laterally, blowing a slab of wood off the left side of the stock (from the rear action screw to forward of the chamber). The remaining gas, moving rearward through the action, sheared the roll pin holding the bolt release button...blowing the button out the left side of the action and cracking the 1/4" plexiglass window alongside the booth. The wrist of the stock broke at the rear action screw. The shooter only sustained some minor punctures from wood splinters. We salvaged the scope and were able to beat the bolt open with a mallet.

That rifle (or what was left of it) hung over the entrance to the range for years. Might still be there for all I know...