Coincidental (adjective)

1. Occurring or happening at the same time by chance.
2. Not planned or arranged.

Origin:

Late 16th century (in the sense "occurring or happening together"): from latin coincident- "falling together", from the verb coincidere, from co- "together" + incidere "fall".

Examples:

1. It was a coincidence that we both decided to go to the same restaurant.
2. The coincidental meeting of the two old friends was a happy surprise.
3. The coincidental discovery of the hidden treasure was a lucky break.
4. The coincidental arrival of the ambulance saved the man's life.
5. The coincidental timing of the flight was perfect for her schedule.