This is a very simple thing photographers should be aware of when taking photos, but it is so easy to be distracted with your main subject you forget to check the rest of the frame for mergers.  The most common places people would notice mergers would be something like a tree or pole coming out of some ones head or things cutting into the edge of the frame.  Sometimes mergers can not be avoided, but more often than not a simple changing of your position or that or your subject can solve the problem.

In these examples I had  simply moved myself slightly I could have greatly improved these photos by avoiding the presence of unwanted intrusions into the frame

Here are a couple of examples of how I improved some of my travel snapshots by moving around a little to get rid of the things that were merging with the main subject.

The key to avoiding mergers is of course to pay attention to what is happening around your main subject before taking the shot

Other Posts on Composition
Simplify Your Subject
Rule of Thirds
Leading Lines and S Curve
Symmetry & Balance
Cropping
Composition Printable