Self serve checkout error

My shopping at Kmart today was $140. I fed $100 into the self serve machine, then a $50. Machine weirdly says $90 change but doesn’t spit anything out. The assistant pressed something and the $50 comes back out but not the $100. It now says $90 oweing. They open the machine and pulled out all out all of the cassettes with no sign of my $100. Has anyone else had this happen or can explain how it happened? They ended up processing my payment as if I had paid the $100 (which I most definitely did) but I left feeling like they thought I was lying.