You'll project your own shortcomings onto each other, getting mad at each other for possessing the qualities that you like least in yourselves. You may even start to represent something to each other that you won't like or respect. If your friend sees you behaving irresponsibly, for example, it'll upset them, because they want so much to be a responsible person.
They could even go so far as to try to force you to behave how they want you to behave -- but that won't work. You'll just rebel against their attempts to control you, which could lead to indiscriminate behavior on your part in the name of pure rebellion. But that won't solve anything, will it? If you feel it's worth it and you want to maintain this friendship, you'll have to struggle to learn to let each other be the people you are.