Attention, memory and perception
Change blindness
People often fail to notice a change on screen, even a big one, if it happens while their attention is elsewhere or without a visible transition.
When a page reloads or a panel swaps instantly, the eye has nothing to catch. People can miss that a total updated, an error appeared or an item moved.
Draw attention to changes that matter: animate the transition, highlight what changed briefly, and keep the change near where people are looking.
In practice
A cart total that flashes softly when a discount is applied, so people see the price has changed.
Common mistake
Assuming a change was seen because it was on the screen afterwards. Watch people in usability tests at the moment it happens.
Where it comes from
Ronald Rensink, Kevin O'Regan and James Clark's 1997 flicker experiments, and Daniel Simons and Daniel Levin's 1998 study in which many people failed to notice that the stranger they were talking to had been swapped.