We had a copier engineer once...
...who argued we had the wrong unit. HIs paperwork had a different serial number to the one on the device and he refused the service. It went on for weeks and weeks. I had to call them up about it (and I'm nothing to do with the IT, just happened to be the office manager) and even on the phone they were adamant it was the wrong machine.
'That's for a college in Stockton' they said, 'Have you moved the machines around without telling us?'
'No' says I, 'for although we too are a college we are different institutions and have absolutely nothing to do with each other'
'Are you sure you're reading the serial number correctly?' they respond.
'Yes. Is it possible I do actually have this machine in front of me and your records are mixed up, or are you suggesting I just happen to have randomly read out a matching serial number for a machine that matches that of a different college you supply?' Which sounds more plausible to you?'
I even had to take a photo and email it. To this day I still remember the Fujitsu PoS and the plate beneath the glass.
Oh, and their asset tag matched up, just the serial number was recorded incorrectly.