My mate chose the wrong coin
If the spec chooses the wrong coin, I say
"Dont worry , I will keep that, your chosen one, safe for you and nothing will happen to it, however I can't guarantee what might happen to the other one" to stop them dwelling on it I then let them know that what's about to happen isn't likely to hurt (much), but if they feel anything uncomfortable happening they should ask me to stop. That gets their apprehension up as really they don't know what's going to happen.
slight marking on the coin left behind
The force is travelling in the wrong direction Luke. Do it slowly right in front of your face and you'll see why and which way you need to send your force.

I did it a 100 times
Also doing it within the same group of people a few times, perhaps friends down the pub, I strangely find that it isn't doesn't always happen to the same coin. That keeps them guessing.