After the snap, a B player comes on the field but does not participate in the play, which is a dive up the middle. Since I'm on B's sideline, I flag it - he wasn't just a step or two on, but he didn't get close enough to the pile to hit somebody or anything. We enforced it from the previous spot and replayed the down.
When I was in the car driving home, it hit me that we may have messed that enforcement up. Since it was during a running play, shouldn't we have enforced that from the end of the run?