I've had it happen a couple of times. On my Dillon 650 I notice that the "down stroke" doesn't take much effort. I merely let the case progress, remove it from the bullet seating position, pour the powder back into the reservoir and remind myself to be more careful. In the past I have actually seated a bullet but they don't sit far enough into the case to stay there long. Makes a mess when they fall into the finished cartridge bin.
I've got so much 9mm brass now I've stopped picking up range brass other than my own so I don't have the problem anymore.