This one is more a case of finessing an image, but one which it's really important: separating elements of an image. In the image below, one of the critical factors for me was to ensure that the street lamp wasn't overlapping the edge of the buildings. I'd already framed up this photo and played with the composition - positioning myself low to fill the frame more with the bike, and including the flowers and the posters. However, it was only when I moved a little further left, that the street lamp gained the space around it, helping the image to breathe a little more.
You often come across these issues if you photograph anything where the horizon is a strong feature. If something only just intersects the horizon for example, tree branches from above or posts from below, the image is quickly spoiled. By moving the camera up or down you can often solve the problem. Otherwise, if you can't do much about it, try and cross in a way which shows that it was deliberate. There's nothing worse than just the end of something ONLY JUST touching or crossing a line such as the horizon.
Here's an iPhone image taken early one January morning in Venice. I had to hold the phone above my head to take this as the posts sticking up from the water were crossing the sea horizon and overlapping the buildings on the island in the background.
Hope that helps someone. You'll see good photographers do this all day long. You'll now notice the ones that don't, and appreciate just why it's not a desirable feature composition-wise.
Ciao - Andy
P.S. Sorry about the terrible compression on these - it seems to have happened after they were uploaded.