Some years ago we had fun with a client who had systems that insist staff reference numbers are zero-padded, but users like processing data via Excel before uploading for us to import or otherwise process. Even just opening and accidentally saving (habit, accidental change followed by undo with enough time between for auto save to kick in, etc) when intending just to open to inspect, will step the leading zeros off in a CSV. So all code dealing with their manual inputs or bulk imports needs to be aware that those codes may or may not have leading zeros so cannot be used as simple string values as they should be. For extra fun, in one case after a company merge/purchase some codes were padded to a different length so treating them as numbers or unpadded strings internally and padding with zeros for display/export/other wasn't as clear as you'd think.