A good rule when making any photograph is “watch the background.” The warning here is against getting so focused on our subject and its presentation we forget to look at the background and we end up including objects and stray light we only see later. A classic example is a tree appearing to grow out of someone’s head in a portrait.
For my landscape photography, the background decisions to be made are usually how much in or out of focus to make it and to watch for obnoxious highlighted areas. Big landscapes I want sharp front to back, from near to “infinity.” Smaller subjects, like a flower, I might want to effectively remove the background by using a very large aperture. That’s when I really have to watch the background. An unnoticed blop of bright light in the background can ruin the photo.
The attached photo is an example of not paying attention to the background. The subject was supposed to be the plants, their blooms and the tree. When I loaded this onto my hard drive at home, I saw I had a visitor in the photo. I never saw the chipmunk while making the photo. So, I got a harmless reminder to watch my background.