Look in any reputable loading source you have that lists the cases they used to get their data, then look for the paragraph that says something like, 'if you use any components other than what we used, start at the minimum load, or reduce by a given percentage, and work your way up'. There is a whole list of things that if changed will cause higher pressures that most reloaders ignore because the increases usually aren't that great. But if you are loading high pressure, near max., loads, or unwittingly toss in several things that all increase a little, you need to know what you are doing.

How many here inspect their range pick-up brass to make sure the previous shooter didn't drill them out to shoot wax bullets or some kind of shot loads? How many here that never trim their handgun brass also don't bother to check their range pick-up brass to make sure it wasn't trimmed extra short before they found it? How many here didn't know the brass they picked up took a small primer until they crushed a large one trying to seat it?