An interface is modal when the same action can produce different results depending on an active system state or mode.

Modes are sometimes necessary: a drawing tool may interpret dragging differently in select, pan and draw modes. Problems arise when users cannot easily tell which mode is active or when an old mode persists after its purpose has ended.

A mode error occurs when the user performs an otherwise reasonable action under the wrong active mode. The mistake is often caused by hidden state rather than lack of knowledge.

Modes should therefore be visible, have clear entry and exit paths, and avoid changing common controls in surprising ways. Temporary modes can sometimes be tied to a continuously held key or gesture so their lifetime is physically obvious.

When practical, modeless designs or commands that include their target explicitly reduce dependence on remembered state.

The important question is not whether modes exist, but whether users can reliably perceive and control the state that changes action meaning.