Let's say I'm in a meeting and the VP of Marketing says, "We all know this feature easily can be launched next week." At least in the US, my silence could be used later to claim implicit agreement. There seems to be some cultural rule to the effect of, "If someone hears another person say something incorrect and doesn't immediately state disagreement, he is implicitly agreeing or did not care enough about Thing X to bother commenting." If someone believes that others follow this rule, then he could claim that a commitment had been made. At a minimum, it would be easy to raise doubt in others of my competence should I later provide an estimate of a month. So, given these risks, how can anyone not speak up?

I understand letting some non-essential facts slide if there's little threat that they can be used against you later, but US culture (at least Midwestern business culture) dictates near immediate rebuttal -- no matter how gauche.