Accidental (adjective)

1. Happening by chance or unexpectedly.
2. Not planned or intended.

Origin:

Late 16th century: from medieval latin accidentalis, from latin accidens, accident- "happening, event", from accidere "happen", from ad- "to" + cadere "fall".

Examples:

1. The meeting was accidental, we ran into each other at the store.
2. The discovery of the new planet was an accidental find during the telescope observation.
3. The injury was accidental, it happened while playing sports.
4. The painting is an accidental masterpiece, created by a mistake.
5. The relationship was accidental, they met by chance on the street.