Mergers occur when nearby objects or areas having nearly equal values blend together and lose their distinctiveness. The rule for mergers is "don't".
The exception is when being creative, as when you have a child pick up a horse (done by aligning the child's arms in the foreground beneath the horse in the background).
Tonal Mergers occur because objects or areas in close proximity have similar:
Dimensional Mergers occur when objects run together because we lack the depth cues that make it possible to distinguish one object from another.
One example is having a tree that appears to grow right out of a person's head. Mergers like this happen not just because we process objects in three dimensions, but because we tend focus on the subject and block out background objects.
Border mergers occur when objects are cut by the edge of the photo - such as missing feet, missing hands, or missing supports. They also occur when objects are too close to other objects.
You can avoid mergers by recomposing your shot by:
Make a habit of looking at your subject's immediate surroundings as you frame your photo.