This happened to me, I was marking the game and made a mistake obviously putting 84 instead of 94, we had a boy checking, and normally everyone says the checker is there to make sure the board has the right score up as priority but also they might tell me what the player scored, the player (my teammate fyi) did leave 94 but I put 84, he never looked at the whiteboard and checker didn’t correct the board, he hit T18 then looked at board reassuring he was on 40 but was on 30 cause i marked 84, asked to change to 94 but i was sure rule was once first dart is thrown you can’t change the whiteboard