There is a known issue that stems from reusing patron barcodes in Symphony. Until this bug is resolved, please avoid reusing patron barcodes.
In some cases, when a barcode is reused the barcode will remain linked to the old user key, which no longer exists, and the patron will see an error message, "Unexpected Error".
If you notice this issue at your library, submit a ticket with the patron's barcode and the text of the error message.