Question

Please give me an example of a recent major decision (whether in business, politics, etc.) that was made based on assumptions that turned out to be incorrect.

Answer 1

Politics: The tea party republicans chose to stall the passing of the budget bill in US parliament based on the assumption that the the Democrats would cave eventually and give in to their demands (Which included major changes to ObamaCare, amongst other things.)

This assumption turned out to be grossly mistaken as the Democrats held their ground and eventually, the Republicans had to give in quietly.