Sort of similar to OP, I had bought a batch of previously fired Weatherby 7 mm Magnum brass - was likely from an estate. When sorting through them the first time, I discovered that several had head stamp "270 Weatherby Magnum" - they were all previously fired, but not re-sized - a 7 mm bullet slid into the necks of those brass with case head stamped "270". So, either the former owner did not know the difference and fired the 270 in his 7mm, or had re-sized and altered 270 Weatherby brass to work as 7mm Weatherby. Those ones have not been used by me, yet - still in a container, like I received them. And, I noticed - except when I was sorting - and possibly when reloading - but about never when I am out shooting, do I look at head stamps before putting in the rifle - shells come out of the box or container and into the gun.
Is most definitely worth to look and sort, at least once - in some bags of .257 Roberts, was at least two perfect, unaltered 243 Winchester head stamped cases - obviously someone had got them mixed in there.