There is an informal expression 'to get over something', which means that either something annoys/amazes you or you can't forget it or stop thinking about it (most often in a negative way).
He couldn't get over this accident.
We can't get over his sudden decision to leave the country.
His wife cannot get over his having got drunk again.
So, when 'A' said that he saw something he will never get over, I think the other person automatically thought of some annoying/impressive incident that has happened to 'A'. The punch line is that 'A' meant 'to get over' in a more literal sense, which expalins why he will never get over moon - can anyone put themselves above celestial bodies? - No.
That is my take on it anyway. I'm not a teacher, maybe we'll wait for a better explanation.