A lot of seemingly different mistakes that people make can all be explained by them only recognising 3 probabilities: definitely will happen (100% likely), might happen (50% likely), and definitely won't happen (0% likely).
For some reason psych and forecasting studies show humans are inclined to reason this way. But while it's our natural way of thinking, it's a predictor of bad forecasting ability and leads to major misjudgements.
Rounding up to 100% is most visible when people become exceedingly confident about uninformed views. They develop a theory or encounter some evidence, and feel like an issue has been completely settled much too quickly, failing to dwell on how that evidence could be misleading, or at least isn't decisive.
When people are stuck in the maybe/50% mindset, they feel like there's there's no point trying to figure out how likely something really is, because it's so random it just can't be done. This means they miss the chance to benefit from pinning down whether something is actually 60% or 40% likely.
Most importantly to me, when people round unlikely things down to 'definitely won't happen', they neglect that if something will have a huge impact, it could deserve a tonne of attention, even if its probability is only 1%.
Similarly, they become blind to the fact that something just as consequential which is 3% likely, rather than 1% likely, is thrice as important to focus on. Not figuring out the relatively likelihood of unlikely things totally screws with your ability to manage risks by suitably apportioning your attention between each one.