Sitting opposite someone at a refectory table at a National Trust canteen in SE England and both going "OMG! Is that you, Nnnn?!?" We were at Uni in West Wales together 20+ years before. (She got a very good degree, I dropped out)
Went to Christchurch to visit friends. Plans fell through. Drove to Bournemouth. Went into a shop, held the door open for the folk coming out and it was Billy Morpeth and family, my late Uncle's best mate and golf partner, from Co. Durham. Whom I'd not seen for 10+ years.
Going into the Nobel Museum in Stockholm on a spur of the moment whim, stood waiting for the tour, tap on shoulder, someone I'd hired three years before who was doing 48hrs in Stockholm.
My fave - taking the Aged P (who had lived in Sussex since 1948) and my two children to Beamish Open Air Museum. Doing the "Going Underground" trip into the Drift Mine. "Where are you all from?" asks the gaffer. "Sussex" says we three. "Sussex" says the Aged P. "Not with that accent!" says the gaffer. "I wez borne in Sooth Mow-wah." says Aged P. "What's tha' name hinny?" asks the gaffer. "George Nnnnnn" answers Pa. "Good ta met tha'" is the reply. Gaffer carries on down the line. Stops. Turns. "Hang on a minute, tha's niver Jossie Nnnnnn's yoonga brootha?" "Why aye, bonny lad, phawee ya bugga, tha' ken wor Jossie?" "Hway fa slates, thas med me day!" Turned out the late Joe Nnnnnn (my Uncle, as mentioned in Bournemouth point above) taught the gaffer all he knew when the gaffer first went underground at the Craghead Busty, back when I was in short trousers. (All dead now, end of an era. Sobs.)