unscheduled

un- +‎ scheduled

unscheduled (comparative more unscheduled, superlative most unscheduled)

1. Not scheduled; impromptu.
   • 1947 January and February, Gerald Druce (Jun.), “A Journey on the "Slovak Arrow"”, in Railway Magazine, page 17:

     There were two further unscheduled stops, one at a small station, Skalice, where a small party of tourists entrained, and at Břeclav, a junction and customs post on the Austrian frontier.

   • 1962 April, R. K. Evans, “The Acceptance Testing of Diesel Locomotives”, in Modern Railways, page 268:

     Because most diesel failures can be traced to electrical faults, minor in themselves but often difficult to pin-point, any unscheduled halt during a trial run is often the signal for the frenzied unfolding of wiring diagrams and the appearance of an impressive array of voltmeters and circuit testers.