Actual case:

1. British Citizen
2. Holds valid & approved ESTA
3. Has travelled to US on said ESTA before
4. Has also travelled to one of the "forbidden countries" since March 2011
5. ESTA still shows valid as US won't know they have been there, and no automatic reason to invalidate ESTA as there is no link to that country except having travelled there since March 2011
6. As of this time, ESTA website still shows approval as valid, so airline would show passenger as OK to board
7. Passport has entry/exit stamp from forbidden country
8. Pax does not have US visa

What is the most likely scenario if this person travelled today?

1. Airline refuses check in?
2. Airline shows as OK to board, pax travels, denied entry to US & deported back?
3. Airline shows as OK to board, pax travels, allowed entry

Obviously 1 & 2 are bad, with 2 being significantly worse as that kind of thing basically blacklists you forever.

Main difficulty is that no official channels have any information about this at all, so it's all guesswork. Or better to just suck it up and get a visa and delay travel till after visa is obtained?