An event, incident, cost, etc. that was not foreseen.
"The sudden rainstorm caused an unforeseen delay in our outdoor wedding ceremony."
not anticipated
"unanticipated and disconcerting lines of development"
"unforeseen circumstances"
"a virtue unlooked-for in people so full of energy"
"like a bolt out of the blue"
"unseen problems"
Not foreseen.
"The sudden rainstorm was an unforeseen delay that forced us to cancel our picnic."