A violation of separation of powers can result in governmental actions being declared unconstitutional, as it disrupts the balance intended among the legislative, executive, and judicial branches. Each branch must act within its own defined powers and not encroach upon the functions of another.
For example: If Congress were to enact a law granting itself the authority to prosecute crimes, this would infringe on the judiciary’s power and be a violation that could render such a law unconstitutional.