In a hypothetical scenario, you are a VWP national who has managed somehow to arrive at a US entry point, by air, without an ESTA. I know it shouldn't happen. Maybe the airline made a mistake. The reason doesn't matter.
My question is: assuming that there is no other reason to deny you entry to the US under the VWP, can or will the border guard deny you entry simply for not having an ESTA? Do they check? Do they care?
EDIT: As npl points out, there are entry points to the US at Canadian airports, and you can easily reach them without having a valid ESTA. Will they turn you away?