If someone predicts something will happen with a 90% probability, then they should be wrong roughly 10% of the time. We can't determine that from a single event, but we could look at 10 events predicted by the same model. In the case of election forecasts, we would expect 1/10 elections predicted at 90% probability to have the wrong estimate. So we would expect the average person to see it at least once, and probably twice, in their lifetime.