I could imagine that happening at the ticket window in the past. I don't know if it could happen today because the ticket terminals are tied into a computer that I would "assume" locks down everything when the gate opens. Tickets today seem to have a randomly generated barcode printed on them. Similar to how in the old days tickets had five randomly generated numbers/letters code printed on them that looked hard to duplicate.

But who knows...maybe a hacker will get in there one day and defeat that.