The US bombing escalation in Vietnam.

• Prior to the escalation in bombing in the Vietnam War, the Americans wargamed potential North Vietnamese responses in the Sigma I and II wargames. Regional experts were able to almost exactly predict the Vietnamese response, and working-level officers from the State and Defense departments, and the CIA, predicted the actual outcome. William Bundy, the guy running the games, thought the conclusion was "too harsh," and the wargames never influenced actual policymakers. (see H R McMaster's Dereliction of Duty)

(On a related note, something similar occurred with the Millennium Challenge 2002- the Red team used unexpected tactics to pull off unexpected early victories against the simulated US forces, so the general running the war game 'refloated' the sunk ships, then forced both sides to use prescripted plans of action, ignoring the unexpected initial events.)

• The State Department's Policy Planning Council published a separate study in 1964 which essentially also concluded that bombing wouldn't work. Walt Rostow, its chairman, disagreed with its conclusions, so he work to suppress its conclusions; it did eventually influence policymakers, but only after the war had escalated, and even then its conclusions had to be bootlegged out of the council. (see David Halberstam's The Best and the Brightest)