Earlier quoted context omitted.
Keyboard wedge barcode readers have a long history in data entry in industrial settings. One of my current Customers uses the UPC on the finished goods they make to "program" the PLC on their production line with the parameters of the finished good. The barcode reader is just "typing" the UPC and various commands into a serial-to-Ethernet gateway that's ultimately connected to a TELNET server on a PC w/ PLC software…
There are doctors offices where you are supposed to present a confirmation or ID to a patient-facing barcode scanner at checkin. If you let it scan your code at the wrong time, at breaks the checkin flow. I can only assume that a mildly creative barcode could compromise all kinds of things.
I know there are instances of SQL injection out there in the wild that are barcode-accessible. I think there was a DefCon talk about it.