People who are afraid of making a little more money because it puts them in a higher tax bracket. They don’t realize that the higher tax rate only applies to the part of the earnings above the previous tax bracket limit.
Many educated people have a blindspot regarding probability. Some people think that if an unweighted coin turns up heads ten times in a row then it is more likely to come up tails the next time. Similarly, in watching a baseball game, they will say that a certain player is due for a hit because the player has been in a slump. I have been told more than once that a guaranteed winning betting strategy is to double your bet each time when the payback is 2 to 1. The Monty Hall problem is a great example of a relatively easy probability problem that most people get wrong.