Can't say we've ever been told not to!
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I heated a YP railcard yesterday that had a hand-written date on it, the date that was revealed was over a year behind the hand-written date. If I'd had more time on my hands, I would have organised a reception committee, as it was, at the first mention of police the BF bought her a new full open. The passenger herself was quite happy to argue with me that somebody at the ticket office had done it, and she wasn't responsible (despite the date, name, and signature all being written in the same pen, with the same handwriting).
I do love it when somebody in that situation says "Are you calling me a liar?". Their face when I say "Yes, and a fare-dodging thief" is priceless!