So I know that acting without the foreknowledge of those actions is down to ignorance. This has various implications, the main one being that the responsibility of one's actions is not entirely eradicated, though maybe diminished. We may say, 'how could he know better? He doesn't know what he's doing/the consequences of his actions.'. Or maybe the person should have known better not to act because of their ignorance, in which case they still bear the weight of blame, but their actions weren't necessarily bad.
What if however, a person does a seemingly stupid thing, but where they knew exactly what they were doing, or what the consequences would be? Maybe they acted in a way where they wanted to seem ignorant to use this as a pretense for their ultimate aim, but where they don't have to take responsibility for their actions (or at least less responsibility).
Best Answer
In general usage we would say that someone is "feigning ignorance".