shoulder

Hi all! So I'm running some range pickup lake city 5.56 brass through the sizer and I come across some weird looking ones. The sizing die restored the shoulder to the correct dimensions and it passes the headspace gauge. Can anyone help me identify what caused this? Pic is the oddity (left) next...

I have been reloading since 1977. I began reloading .223 Remington in 1979. I have used the same RCBS Reloader Special press since 1979. I have used the same RCBS dies (standard, not small-base) since 1979. They were last adjusted in 2016 to allow interchangeability between a Savage Axis...

I know I'll catch hell for not checking from the get go and loading 100 rounds and then realizing my mistake, but admittedly I made an error when full length resizing my brass and set the shoulder back too far and now there is between 0.009 and 0.010" of difference in the shoulder of a fired...