In the same place at the same time and

not expected - it's the coincidence thing. An author I know has just said she was 300 miles from home on business and ended up staying in the same hotel as her husband who was also on business. She did not expect to see him there. Coincidence.
A cousin of my father's likes to tell the story of how another cousin almost got run over in Oxford Street running to the other side to catch up with him. Neither knew the other was in London. Coincidence.
Book plots can revolve around a coincidence - often 'not trusted' in a crime novel. The author's experience however made me wonder again about coincidences. Was it really that surprising? How likely was it that her husband's business would take him to the place in question? Did he know she would be visiting the same place - or even that she might be? Had either of them stayed in the same hotel before? Is the hotel part of a chain they often stay in? They are, after all, married. If they are happily married, and I assume they are, then their values are likely to be much the same and they will seek out similar locations.
My father's cousin was in London just after the war. Like his cousin he was waiting to go back to Australia. Most Australian visitors go to London and most of those who go to London go to Oxford Street. They both had just a couple of days there. The probability of them meeting becomes much higher when the situation is analysed in this way.
If we analyse a coincidence it would seem that it has to be rooted in probability. There have to be things that make contact or convergence likely. It is surely what makes coincidence in a good plot believable?