Biases in thinking
False consensus effect
Also called You are not the user
People overestimate how many others share their views and habits, which leads teams to design for themselves.
If you find something obvious, it's easy to assume everyone does. Teams who know their product well and use it daily are about as unlike their typical user as possible.
The fix is contact with real users: interviews, usability tests and analytics, before a decision is made rather than after.
In practice
A team of engineers designs keyboard shortcuts first and a menu second, for customers who mostly use touchscreens.
Common mistake
Asking colleagues for feedback and treating it as user research.
Where it comes from
Described by Lee Ross, David Greene and Pamela House in 1977, after students who agreed to wear a sandwich board predicted most others would agree too, as did those who refused.