accidental unintentional
odd, stray, unplanned
unplanned off-the-cuff impromptu
Haphazard is another word for random. Additional synonyms include arbitrary, indiscriminate and unplanned.
Unplanned.
Another word for unplanned expense is "unexpected cost." This term refers to expenses that arise suddenly and were not anticipated in a budget. Examples include emergency repairs or medical bills. These costs can disrupt financial planning and require immediate attention.
This side trip was unplanned, but enjoyable.
ed
UNPLANNED
Random
Probably unknown.
This is likely the word "accident" (an unplanned or unexpected occurrence).
Impromptu is the closest to spontaneous word. It is a word representing sudden action.