Just in case you've forgotten...
 

found this warning on the flap of a vintage shell box:

https://i.imgur.com/sDFNPqgl.jpg

soooo

20's in a 10 are OK?

From a late 1940s/early 1950s Western Super-X 10-gauge box --

Attachment 89709

Yet they sold reloading components.

I think the only issue is 20's in a 12, which will jam up several inches in front of the chamber. I don't think any other combination will jam

I remember a woman shooter friend who accidentally tried to load her 12 gauge with her lipstick.

The main warning I needed one time only was to take the same gauge shells as the gun I take, ended up a long time ago coming home after work, grabbing a gun in the case, grabbed a box of shells, ended up in a public game area with a 20 gauge side by side and 16 gauge shells, Long time ago, no good excuse.

Dont tell me what to do!

It didn't. It told you what NOT to do :)!

How 'bout using Proof Loads??

1933 lawsuit against Remington Arms related to the burst barrel of a Baltimore Arms Co. shotgun after using a Nitro Club marked Proof Load.
https://books.google.com/books?id=7p...C&pg=PA120&lpg
https://books.google.com/books?id=7p...C&pg=PA127&lpg
The “maximum commercial load” was described at “5 long tons or 11,200 psi” (calculated long tons/sq. Inch X 2240 = PSI)
The proof load was “7 1/2 long tons or 17,800 psi”.


1936 testimony by W.A. King, Parker Gun Co. regarding a barrel burst, likely a 20g shell inserted before a 12g
https://books.google.com/books?id=jU...J&pg=PA802&lpg

Met a fellow a few years ago who was sitting on a rock right after other hunters entered the fields on opening day. When I asked him if he was OK, he told me that he had brought a 16 gauge gun but only had 12 gauge shells with him. I offered to give him some of my 16's and went to my car to get them. When I returned with the shells he was so grateful that he offered to " bird dog " for me. Just before I was about to accept his offer he asked me " what does a pheasant look like ". I graciously declined his offer, but would show him one if I was successful.