When a 20-gauge shell lodges in the larger 12-gauge barrel, danger follows at the next pull of the trigger.
Using the correct ammunition for any firearm is an important detail. Always follow manufacturer recommendations about how to use and care for your firearm.
Watch these guys show us the danger of mixing ammunition sizes.
This was an eye-opening demonstration.
The smaller diameter 20-gauge shotshell can push far enough into the barrel so a live 12-gauge shell can be loaded behind it. When the 12-gauge shotshell is fired into the primer of the 20-gauge shell, disaster could occur.
These guys did a controlled experiment with a tough old shotgun and netted interesting results. While the barrel did not rupture, it did bulge from the high pressures.
When shooting a 12-gauge shotgun, always leave the 20-gauge shells at home. Period.