Like a lot of our industrial customers, the specs for her labels call for Code 39 barcodes, which she’s been printing for some time with no issues.
Aha – so now it’s clear!
Our customer had run into the limitation that the regular Code 39 format was designed to encode the number 0-9, the set of upper case letters and a few special characters. Lower case letters are not allowed.
That’s why BarTender wasn’t happy with her, if she added a lower case letter to the data to be encoded, BarTender showed it as an error and printed a 0 (zero) where the illegal character should be.
Simple, really! BarTender (indeed, just about all label software) comes with the option to use Full ASCII Code 39 as the barcode type. The Full ASCII version allows all of the 128 ASCII characters to be encoded correctly so the customer just needed to switch over to that.