I think one of the biggest mistakes is choosing a puppy instead of a rescue dog that is full grown and here are a few reasons why;
1) Most people forget that cleaning up after a puppy is more work than a baby in the beginning, along with the emotions it brings when they have a loose bowel movement on the new carpet, they chew up one of the legs on your favorite chair when they start teething, they pee all over the bed or couch when you didn't get up to take them out. This along with a lot of other examples create a multitude of emotions and problems that most people never even consider.
2) They don't consider how big the dog will get once it is full grown and what caging that animal all day while they are at work will do to it's temperament, (big or small, along with the mess they have to clean up when they get home and the smell.
3) They don't look into how that dog was bred for a specific purpose and if that purpose is something that is going to be allowed in their home or if the dog will live like a human in jail for the rest of it's life.
4) They buy a puppy for a child who promises to take care of the puppy without taking into consideration the age of the child, and that they don't have a clue. You should see what I tell people to do when their child wants a horse.
5) Finally they don't realize that getting a full grown rescue dog who is already housebroken, full grown, leash trained, etc., would relieve them of a world of future problems. It also stops puppy mills from making $$$ and lowering the percentage of their existence. Dogs at rescues (not just the pound) are there mostly due to financial issues, or the owner moving, NOT BEHAVIOR ISSUES.
Sorry to ramble but this is a HUGE ISSUE with me just check out my hub