A password becomes a practiced routine
A password is not stored only as a string of characters. Repeatedly typing it links the characters to a familiar sequence of hand movements. When the same login screen appears, that context can cue the old sequence before you have fully thought through what you intend to type.
This is a form of retrieval competition: when several memories could fit the moment, the one that is most practiced and easiest to call up often comes first. The old password can therefore intrude even when you consciously remember that it has changed. Your hands are not literally remembering on their own; practiced movement and memory work together to make the old response feel automatic.
Why the new one feels awkward
The replacement begins at a disadvantage. You may have typed it only a few times, while the previous password has been rehearsed across dozens or hundreds of logins. That imbalance can produce a familiar slip: you start entering the old password, notice the mistake, and have to stop and deliberately recall the new one.
With repeated use, the new sequence becomes easier to retrieve and the old one loses its advantage in that setting. Until then, pausing to recall the replacement—or using a password manager to fill it in—can interrupt the well-worn routine. The first few logins are not a reliable test of whether you have forgotten the new password; they may simply be the point when the old one is still the easiest answer to produce.
