Attention, memory and perception
Selective attention
Also called Inattentional blindness
People focus on what matters to their task and filter out the rest, often missing things that are plainly on the screen.
Someone hunting for a button can look straight past a message about why it's greyed out. Attention follows the goal, not the layout.
Put information where people are already looking, at the moment they need it: an error next to the field, a hint beside the action.
In practice
A form that shows an error at the top of the page, while people's eyes are on the button at the bottom, leaves them clicking it again and again.
Common mistake
Assuming people saw something because it was on the screen. Test whether they noticed it, not just whether it rendered.
Where it comes from
Colin Cherry described the cocktail party effect in 1953. Daniel Simons and Christopher Chabris's 1999 study, in which half the viewers missed a person in a gorilla suit, made inattentional blindness famous.