Thanks KAR, I searched but didn't find that thread. I note the one actual incident in the thread:-
If for some reason your flight to Canada is diverted to a US airport and you don't have an ESTA, upon reaching the nice person at Immigration and you are from a visa-waiver country, they will give you one of the green I-94W forms (the ones you used to fill before ESTA) to fill out and then process your entrance to the USA. At least, that is what happened to me in 2012.
and I will go merrily on my holiday in the expectation that this still applies.