I've done this more than once. Usually for something I critically needed to survive. Going back is usually not an option either, at least not in the same day because doing so would acknowledge your failure too deeply. One must wait at least a day so the cashier is different and doesn't recognise you. At least it's only ketchup, though. The higher order version of this scenario is the camping trip and the one thing you absolutely needed that you meant to pack and set aside so you wouldn't forget but... forgot.