Eftpos machine says swipe again once paid..?

so i havnt been working at my job for that long but i know all the ropes and everything and this has happned three times now and the boss is pissed off. so i serve this man and finalise it like normal and he wants to pay on eftpos and that goes through normal to then i asked if he wanted any cash out and he said $20 and i put tthat in the screen like normal then i was about to finalise everything and print the receipt when the little screen came up for eftpos again and i was confussed but the man just swiped and paid again because i thought the card must not have worked and needed another go. then when i typed in how much money he wanted again it worked but then the screen went ****** and said he owed whatever the total was and i called the supervisor and she just typed in the total and it wouldnt let us do anything! it was really weird and look like half an hour to fix and the man ended up getting charged twice. i seriously have no clue what happned has this ever happned before? or do you know whats wrong!? my boss is pissed with me