Shake

A shake is a refusal expressed as motion. The control moves a few pixels left, right, and back, two or three times, in under half a second, and lands exactly where it started. Nobody has to read anything to know what happened, because the gesture is the human head shake with the head taken out, and it arrives before the eye has finished travelling to any text that came with it. The macOS login window made it famous, and the pattern has been copied into every passcode screen since.

The motion has strict limits. It has to be lateral, because a vertical wobble reads as a bounce and a bounce reads as playful. It has to be short, because anything past roughly 500ms stops being a reaction and starts being a complaint. And the amplitude has to decay, so the last pass is smaller than the first: a wobble of constant width reads as a mechanical loop rather than as something recoiling. Because it returns to its origin and moves by transform, it costs no layout at all and nothing around it shifts.

A shake is a reaction, not a status. It happens once, at the moment of the rejection, and then it is gone, so it can never be the only thing that says an attempt failed. Pair it with text that stays: an error message beneath the field, or inline validation that survives long enough to be read. Motion is unavailable to anyone not looking at that part of the screen, and it is switched off entirely for readers who ask for reduced motion, which is the correct behaviour precisely because the shake was never carrying information the words did not already carry.