The world of action pistol is now shooting "9mm Major."
This is 9x19 loaded to VERY high pressure (to take a 145 max power factor round to 165+ power factor).
So high, that the shooter uses "once fired" brass (who really KNOWS how many times it has been fired unless from military or police use?) that is cheap and LEAVES it on the ground from what I read.
You pick it up and you are using brass that has been ABUSED. I first saw "abused" 9x19 cases about 3 years ago. They passed my eyeball inspection for bulges, but they produced a raised ridge of brass near the extractor groove (see the Lee Bulge Buster ad and picture/drawing) than prevented the rounds from chambering. Since this just started 3 years ago, and I have found the occasional case since, and have not seen it before in about 37 years of reloading 9x19 previously, I "assume" it some new cause.
If the loader used a proper slow powder, the case is not too badly abused (I know that a top load of 3N37 and a 124gn jacketed bullet in Lapua manual will get you to 154 power factor and I assume that it meets at least European pressure standards--which are still, I have told, above SAAMI pressure). If the loader used a common and cheaper fast-to-medium speed powder, you could be using a case that has seen 50-65ksi.
Thus, if you pick up brass, you need to inspect it carefully (since any cartridge can be abused) and use it at your own risk. NOBODY can tell you it is OK to use. Everybody has to warn you or else they could find themselves liable for any damages.
This is all parts of living in the modern world. Haven't you run into this any where else?