In a bag of Winchester 264 Win Mag brass (factory sealed bag) I found two
(unprimed) 300 Win Mag ... not terribly hard to mix them up.
Only noticed when I went to FL size them before use ...
and yes, I FL size, trim, debur the case mouth and primer flash hole, uniform the primer pockets, tumble & weigh/segregate all new brass cases before use. Removes a lot of variables !