Skeletons and Ghasts can keep their bow/fire charge 'charged' after losing target on a player, and then fire immediately after the player walks back into their field of view.

When in combat with a Skeleton, if you walk away far enough for them to stop targetting you, and then walk back into their sight, the Skeleton does not have to prime their bow again before firing, instead using the residual charge they had built up from preparing to shoot you before you walked out of their view.

For skeletons this is very noticeable as they can “no-scope” you if you try to ambush one after they've already seen you at least once. The skeletons will still move at normal speed whilst their bow is pseudo-charged.

In the ghast's case, the ghast will stay stuck in the firing animation until it can see you again, at which point it will fire its fire charge after finishing the firing animation.