Biases in thinking
Mental models
The picture people carry in their heads of how something works, which they use to predict what a product will do and what to do next.
People don't read manuals; they guess from experience. If a product works the way they expect, it feels intuitive. If it doesn't, they make mistakes and blame themselves or the product.
Research how people think about the task before designing it, and match their model where you can. Where you can't, show them clearly how this works instead.
In practice
People expect deleting a file to move it to a bin they can restore from. A product that deletes permanently without warning breaks that model.
Common mistake
Assuming users share the team's model. The people who built a product understand it in a way nobody else does.
Where it comes from
Psychologist Kenneth Craik proposed the idea in The Nature of Explanation (1943). Philip Johnson-Laird, and Dedre Gentner and Albert Stevens, developed it in books both titled Mental Models in 1983, and Don Norman brought it into design.