My main camera is a Nikon D700. I love it. Especially the auto focus feature. Most of the time.
One problem. It has three settings; manual, continuous and single.
I like to use the single setting. That way I can focus on an object, hold the shutter release button halfway down, recompose, and know that the important subject will be sharp in the final image.
But it’s easy - real easy - to accidentally move the selector lever to continuous. Which mens that the autofocu is working whenever the shutter release button is pressed. Thus, when I think the prime subject is sharp, it may well not be. Case in point below.
Lesson learned; check ALL those little switches and levers all the time.