My fav "abuse" of the system was a car park terminal that was running some flavour of Windows with an antivirus software.
It had a scanner for the barcode of a ticket, but, it understood lots of other barcodes/encoding systems and must have been logging to the filesystem.
So... saw someone encode the EICAR test string to a QR Code and put it to the scanner... that caused the AV to popup which covered the entire screen and made the terminal unusable!