William James named the Psychologist's Fallacy in the 19th century. It is a type of fallacy in which someone who partakes in an event assumes that his/her experience reflects the true nature of an event.

This fallacy may also be about two people - the observer and the observed, rather than an observer and a fact. This interpretation of the fallacy means that an observer assumes that someone else will respond to an event in the same way that he/she would in the same circumstances.

An example of this fallacy in the observer-event interpretation would be,

"I didn't think that roller coaster was very fun. That means that the roller coaster is not a fun ride."

This is an example of the Psychologist's Fallacy because the person who rode the attraction didn't enjoy it and therefore assumed that his/her interpretation that the ride is unenjoyable is the objective truth. That may not be the case, as one's enjoyment of something is subjective. There's a large possibility that someone else could like the ride.

An example of this fallacy in the observer-observed interpretation would be,

"Climbing that mountain was a piece of cake. I'm sure Marjorie would have an easy time doing it as well!"

This is an example of the Psychologist's Fallacy because the mountain climber assumed that just because she had an easy time climbing the mountain, her friend would also have the same experience. Once again, the person thought that his/her own experience was the definitive truth and no other possibilities could exist. It's a possibility that Marjorie could've had a hard time climbing that mountain.

An example of this fallacy in film would be in Jurassic World (2015). In one scene, Claire and Masrani are looking at the Indominus Rex's enclosure through a glass window in a control room. Claire ensures Masrani that the enclosure is safe and that the park has the best structural engineers in the world. Masrani replies, "So did Hammond." Hammond was the founder of the first Jurassic Park, where things went awry many years ago. This is an example of the Psychologist's Fallacy because Masrani assumes that the park isn't safe just because Hammond had the same experience many years ago and it wasn't safe then.

Here is a video explaining the Psychologist's Fallacy with plenty of examples.