It is not uncommon to encounter the unexpected. Who really knows what the next moment will bring?
This can actually be rather satisfying when going after a shot. Frequently, we zero in on a subject and work on crafting an image. We do what we can to manipulate the variables, playing with angles and timing to get it the way we like it. The more we can focus, the more intent we can be and the more powerful our photos.
As this is happening (and it should) other visual opportunities often may arise. People and object may move about or we may suddenly see something we did not notice at first. Grab the shot.
It is rare that you will regret taking a photo you do not need.