I had a fun experience in my local shop yesterday. A guy came in with three pieces of fired .223 brass. All three had the primers blown back just a bit, and one was split just about 1/4" in front of the head.
He said that these were the first three rounds he had fired in his brand new Marlin rifle. The guy behind the counter just kept saying that the only thing that could cause that would be overcharged ammo. I asked him if he had reloaded it, and he hadn't. It was factory ammo from two different manufacturers (Remington and Winchester).
I told him it was a headspace issue. Then the shop owner decided to call one of the guys he knew at Remington's service center in Illinois (shop owner used to be a sales rep for Remington). The phone call confirmed what I told him, and that rifle is going back to be rebarreled.
Before I mentioned headspace, the guys at the shop had just about convinced the guy to go home and try firing it with some Federal ammo.
While I've now read a couple different reloading manuals, it was this thread that originally taught me about headspace. So our discussions on here are helpful to the safety even of shooters who've never heard of TFF.