Needles and PINs

When I was on shift at the vet the other day I was horrified slightly alarmed to see that the receipt roll for the credit card machine was about to run out and I've never changed it before, and had no idea where the destructions instructions were. I toyed with the idea of leaving it for the next person on duty because there weren't many appointments booked, but that doesn't mean it'll be a quiet shift sales-wise so I didn't dare. I cautiously opened or removed all flaps and covers and thought it looked reasonably straightforward, so I got a new roll ready (I'm not entirely stupid), took a deep breath and removed the remnants of old roll. I carefully fed the the end of the new roll through any appropriate-looking slots then panicked as I realised it needed mechanical assistance for the final stretch. How?*

A brainwave! If I used my own credit card and made a dog's breakfast of the sale it might still feed the paper through. I duly inserted the card, logged a sale for £0.01 and pressed random numbers because I have no idea of my PIN. This would ensure a void sale, I'd be asked to remove the card, the recipt would come through and all would be hunky-dory. What are the chances that the random numbers I pressed were right?

I left a note explaining the strange 1p sale on the receipt (yes, it worked. It seems I do know my PIN after all) in the cash box, which today was found annotated by BossVet "What the !!!!? See me"

Oops!

*If I'd been looking at the machine the right way up I'd have noticed the button marked 'Feed'.