When I first started loading, I wanted to load .357 and .45 Colt. Well, I couldn't find components for either at first, but finally managed to buy 200 brand new .38 special cases and small primers. I figured out that with those brand new .38 special cases, I didn't need to size them at all. Of course, I figured! They were brand new so they were perfect, thus saving a step. Everything went very well and I got to load quite a bit like that.
Finally though I was able to secure a precious bag of 100 brand new Winchester .45 Colt cases and 300 Large pistol primers and a set of dies and at long last I was ready to load up some .45 Colts for my revolvers. Being the old pro that I was, I knew there was no need to run those brand new cases through the sizer die at all, so I proceeded to prime, flare, and charge the first 40 rounds. The problem came when I tried to seat a bullet. The first one dropped right through the mouth and into the powder. Hmmm...that was odd. The second one was barely able to sit in the case mouth so I put it into the seater die and pulled the handle. Again, I was left with a bullet sitting way down inside the case, this time with a crimp on the mouth. I did that on 2 more before I got the excellent idea of giving them a bit of tension by taking a pair of pliers and purposely bending the mouths in just a tiny bit to give some tension. It worked OK I guess. By the time I seated a bullet and crimped them, the mouths were good to go. I had figured out by then that even those new cases needed to be sized, but I was too naive to know that I could put primed cases through the sizer die if I took out the decapper pin. I guess it's a relic of when traditional diameter for .45 Colt was .454 rrather than .452.
Ever since then I put every case through the sizer die. New cases just get the necks sized, but I don't assume that factory brass is perfect anymore. But I sure was in a sweat to think that I had screwed up my only .45 cases that I had found so far.