When making a retaining wall you will almost certainly need to make a corner.
You can either make your own or buy them pre-made. I prefer to make them myself because I can control the quality. The pre-made ones are made with the same tool I have at the masonry supply yard by their guys. Sometimes they rush it and one corner is bigger than the other or the corner is not really 90 degrees.
Some contractors do not know how to do these or they don't have the tool and do not want to pay for corners and they make something that looks like this.
Notice how unattractive the verticle cuts are at the corners, these are two blocks that are mitered together. You may also notice that the contractor did not control the bond lines running vertical in the wall, these are very weak spots. Bond lines are the joint where one block meets another. You should never have one on top of another one, Here you have up to 7 stacked up. This wall will fail no doubt. A local contractor was so impressed with his own work that he posts it on his web page.
Below is one of our jobs, Notice the nice corners and staggered bond lines in the step treads.