For all we know, the traders could have flagged it minutes after it was placed and the guy had already left the shop - we only have his testimony for how it happened (and as mentioned above, he's said other things that raise red flags).
Similarly, if he paid cash there's potentially no opportunity (whether intentional or not) for the shop to rectify it until he comes to collect his "1495/1" winnings. So they have paid him out - considerably more than he's legally entitled to under the terms and conditions - just not at the odds he invented.
Given Argentina + Messi POTT was being offered around 20/1, that would mean France are 70/1 to be the losing finalist in a tournament they were 7/1 to win... nobody is buying that argument, and a competent adjudicator will quickly toss such a case in the bin.
Depending on the error, yes you could - e.g. if you wrote a lower price on the slip then they should honour it at the correct price, or if you write a mathematically impossible bet (e.g. an accumulator where both teams win the same match) then they should void the bet.
Admittedly not all bookmakers would though, which is a problem - which is why it's important for the media to amplify the genuine miscarriages rather than cases like this where the guy is clearly trying it on.