Alice is playing against Nicky in round one of a Standard PPTQ. Alice controls two untapped Plains and two untapped Mountains. Alice taps both Mountains to cast Veteran Motorist and announces she will scry 2. Nicky confirms. Alice looks at the top two cards of her library and places both cards on the bottom of her library. A spectator watching the match notices Alice forgot to tap one of her Plains when she cast Veteran Motorist and calls for a judge.
Judges feel free to discuss this scenario on Judge Apps!
Although those steps to fixing the error follow tournament policy, the Knowledge Pool team agreed that the game should not be backed up in this case because Alice had the opportunity to manipulate cards in her library. Despite the possibility Alice would follow the same line of play as before, there is risk that she could change her decision by choosing to not immediately re-play the Veteran Motorist, and still know the top two cards of her library.