I started reloading shotgun in the mid-seventies. I used a friend's Lee Load-All. I didn't know, or care, what was going on, as long as I put hull here, pulled lever, set primer, moved hull, pulled lever...it was brainless. Never had an issue.
Started reloading rifle summer of 1981. Used a Lee Loader. Worked great. Never had an issue until I had real presses. Loaded rounds for hunting. Went hunting, chambered first round, seemed really tight and wouldn't close. Tried to extract and couldn't. Rifle was useless. Day was over. Went to a friend's (mentor, actually) house with it, explained what I had done, and learned about case spring, hardening, and annealing. I also learned how to get that stuck round out.
About 15 years ago, I made up some rounds that were set up as just off the lands. The work-up rounds went great, however, when I made the batch of 50, I somehow switched (or otherwise got wrong) a couple of numbers on the COL and when I chambered the first round, it was hard to close (I ALWAYS try all the rounds now in the basement, BEFORE I hit the field). Fearing that I had somehow duplicated something I had not done in many years, I attempted to extract the round. Surprisingly, it extracted with little effort but then powder was everywhere. I got the case out, sans bullet and powder. The bullet was still in the rifle. When I tapped it out, I realized my mistake because the rifling was HEAVILY engraved into the ogive. Checking the OAL of the rest, I wound up re-seating all of the remaining 49 to the proper length.
So, stuff can happen, esp., when you don't pay attention.