mepal
I’m also Dr. Pepper but not the one you are thinking about. I’m substituting for my husband for the time being. (I’ll let him know you miss him.)
I just consulted my husband (or “The Doc” as he likes to be referred to as) and he explained the scenario in somewhat more detail.
The mishna in Makkos Perek Gimmel Mishna Yud brings down the different opinions on the number of lashes on receives- either 39 or 40. When will it not make a difference?- when the messenger of Bais Din who is administering the lashes already deserves 39 sets of makkos for prior aveiros performed. While he is giving lashes to another person the counter counts to 39 and gives him a warning not to whip him anymore. He continues with one more whip.
So if the correct amount of lashes is 40 then he did nothing wrong and he is still liable for 39 sets of 40 lashes (1,560 ouch!). If the correct amount is 39 then he is now liable for an extra set for wounding someone, and now he is liable for 40 sets of 39 lashes (also 1,560).
My question above “Why is someone who does so many aveiros sitting on a Bais Din giving lashes?”- the mishna in Sanhedrin, perek tes mishna hay says that if one is liable for two sets of makkos he gets put into a cell and fed barley- not appointed to bais din to administer makkos?