The typical mind fallacy is the mistake of modeling the minds inside other people's brains as exactly the same as your own mind. Humans lack insight into their own minds and what is common among everyone or unusually specific to a few. It can be often hard to see the flaws in the lens, especially when we only have one lens to look through with which to see those flaws.
The typical mind fallacy is also accompanied by the atypical mind fallacy - the idea that no one has the same mind or thoughts as you and you are unique.
The typical mind fallacy can usually be found making other fallacies worse. For example, causing biased and overconfident conclusions about other people's experiences based on your own personal experience.