This has happened four times to our family.
Walking back into Florence city, having visited a well-known market on the outskirts, our elder daughter noticed her Design Technology teacher on the corner of the street. She had a short conversation with her.
Another occasion, eating at a pizzeria outside on the terrasse in Duras in France, the same daughter saw a school acquaintance walk up the street and duly caught up with her to talk briefly.
Third time, my husband and I were walking in the square in Sonoma, California and someone called my husband's name.