For clarification, it's possible because the classic turret primes on the down stroke of the decap/size operation. Priming itself on the classic turret requires more of a down stroke than is required for the rest of the operations. There's not much feedback on the priming portion, and I'm a noob so I had only been checking the primer seating after the rounds were completed.
But thanks to this incident, I've added another step. Check the primers!