Alan Cooper

to minimize mouse movement

When a control actively rejects keystrokes during the entry process, this is an example of active validation. A text-only entry control, for example, may accept only alphabetic characters and refuse to allow numbers to be entered. Some controls reject any keystrokes other than numeric. Other controls reject spaces, tabs, hyphens, and other punctuation in real time. Some variants can get pretty intelligent and reject certain numbers based on live calculations. For example, numbers might need to pass a checksum algorithm.

If an explanation is offered, users will be less inclined to assume that the rejection is arbitrary (or the product of a defective keyboard). They also will be in a better position to give the application what it wants.