I would be surprised if there is anyone who has never been surprised by something that was caught in a picture that was not planned, maybe not even noticed until post processing.
When I have time (?!), I plan to read a book on exactly this subject: "Photography and the Art of Chance"
Has anyone read it?
To start, I'll post this picture in a Paris restaurant this summer where I saw that the mix of setting sun and indoor light looked appealing, so, I just turned in my chair for one, only one shot and a waiter blurred perfectly in the corner!
_170806_130942_DSC2441 by BG Day, on Flickr