Technically, unless it can be proven that you were intentionally
waiting and watching a machine to jump in and steal someone's
voucher as soon as they left it than you really can't be charged.
There do seem to be a couple of these "lurkers" in most
The betting site will always try to do the right thing. So they'll lock a ticket and track it down if at all possible. They'll do their best to get the money back to the right person, it's good pr.
But in reality it's a stretch to have someone arrested over a voucher incident like this. I've seen many cases where a person doesn't even realize they've added someone elses voucher to their own. Bettors who pound a lot at the window aren't always sure their exact total, and i've seen a few in my days that are completely upset unitl shown the betting logs what happened and how they ate someone else's voucher.
In the end, possesion is 9/10's of the law. Unless the person was a known 'lurker' prosecution will be tough, even if an arrest was made. If it's true that the voucher was in the thousands, and the person immediately grabbed it, ran to the window and cashed it, that might not look good for them in terms of prosecution, but still, the bettor is responsible for removing their voucher at the end of each session.
If a teller had accidentally given them too much money, it would be the teller that is down the creek.