I had a $2 scratch ticket that won a Free Ticket. The store clerk says it is not a winner, tears it, and doesn't give ticket back. I asked for the ticket back and was given another ticket (same game) which was a non-winner. After saying that this was not the ticket I handed the clerk and describing where the 3 winning symbols were located on the ticket, she finds the ticket and hands it back to me.
I know it is a winner because of the 3 symbols and the "TIC" three letter code on it. On the way home, I stopped by the regional lottery office which is a few blocks from where I live. They run the ticket and find it is a winner and has not been paid yet and they give me another new ticket. This also means it was not scanned at the store.
They take the store information and write them up for additional training.
I don't think the store clerk did it on purpose, but you never know. The amount won was only a free ticket (or $2 credit to the store).
It is a good idea to always know if your ticket is a winner before handing it to a clerk and to get a supposed non-winner back from the clerk (if someone claims your ticket is not a winner, it is still officially your ticket and you have the right to get it back).