Running a 9x18mm case through a 9x19mm Luger sizing die does nothing earthshaking. I had one mixed with some 9x19 brass and did not recognize it until I had charged the powder and placed it in the loading tray at which point the length difference became obvious.
Since I use a single stage press and am unswerving in checking power levels before stuffing a bullet in a case, I probably would have noticed it when I checked the powder level. The length difference also would have definitely been obvious when a bullet was seated in a 9x19 seating die setup.
I now check the length of all 9x19 brass i plan to load and have found a few 9x18 cases in range pickups.
However, the danger of 9x18 brass (or 9x19 headstamped brass shortened to 9x18) not beilng recognized in a progressive press loading 9x19 rounds is significant.
Even though the 9x18 operating pressure is less than the 36,500 psi of the 9x19, I would expect at least some primer setback if not pierced primers when a cartrildge 1mm too short is fired in a 9x19 chanmber, not to mention feeding dlfficulties and other potential attendant problems.