Unfortunately, I have had the other experience. Sat down behind another tech, to complete crossmatches, pulled cards out of centrifuge and they were completely compatible. When I looked at the cards for the pt type, the patient was B pos. Tech had written A pos and crossmatched A pos units. I can only pray that this would have been caught, with all of our other checks in place before the pt got the blood. But this was enough to convince me.