Biases in thinking
Confirmation bias
People look for, notice and remember evidence that supports what they already believe, and discount evidence against it.
Once a team believes a feature will work, every positive comment stands out and every doubt sounds like an exception. Users do the same with products they already like or distrust.
Guard against it by writing down what would prove you wrong before you look at the results, and by asking research questions that could go either way.
In practice
A team reads usability sessions looking for proof its new flow works, and files every struggle under 'participant error'.
Common mistake
Telling users they're wrong. Arguing against someone's belief head-on tends to entrench it; start from what they already think and show them something new.
Where it comes from
Peter Wason demonstrated it in 1960 with his 2-4-6 task, in which people tested only the rule they already suspected.