Partcipants were asked to predict the outcome of the votation for supreme court nominee Clarence Thomas
Before the votation 58% predicted Thomas would be confirmed. After the sucessful confirmation, 78% claimed that they thought Thomas would be approved.
We tend to look back at an unpredictable event and think it was easily predictable.
See the full study here :
Dietrich, D., & Olson, M. (1993). A demonstration of hindsight bias using the Thomas confirmation vote. Psychological reports, 72(2), 377-378. https://doi.org/10.2466/pr0.19126.96.36.1997