Just in case you've forgotten...

[Big D]

I have a M21 20 gauge two barrel set. I thought I had put the short barrels on the gun, but when I got out in the field and tried to shoot a grouse, the gun didn't fire. Long story short - I had put the 20 gauge barrels on my 12 gauge frame and never noticed the width difference. Gun locked up tight, but the firing pins were no longer centered on the primers

[Keith Doty]

One of my customers had a guest in S. Texas last year that ended up with a 28 jammed in a 20. ALL BAD. Gun and left hand badly damaged. Another hunter in the rotation picked up the wrong gun, dropped 2 28 ga. into it, realized he had the wrong gun, dumped the shells out( he thought), they fell in deep grass. In his haste to get to the point, he evidently didn't confirm the barrels were clear and put the gun back. He only found 1 shell in the grass.
I have a life long habit of looking down the barrels just before stuffing shells in. It just takes a second.......

[charlie cleveland]

that's a good habit to have....some times some of us fail to look at what shell we are trying to put in our gun....I one time grabbed the wrong shells for the hunt just as I was going to stuff shells in the old 12 ga double I relized they were 16 ga...lucky but it happens...easy to do.... charlie

[Bill Murphy]

In a pinch, I have used 12 gauge shells in a ten gauge gun. I would only do it if I were out of tens.