Unscheduled (adjective)

1. Not planned or arranged beforehand.
2. Unplanned or unexpected.

Origin:

From the latin word 'inschedule' meaning 'not scheduled or arranged'.

Examples:

1. The unscheduled stop was necessary to pick up a forgotten item.
2. The unscheduled meeting was called to discuss urgent matters.
3. The unscheduled arrival caught everyone off guard.
4. The unscheduled maintenance shutdown caused inconvenience for many customers.
5. The unscheduled snow day allowed students a day off from school.