What happens if you over charge a ac system?

Usually there is no immediate result except for reduction in cooling proficiency. If the system is overcharged it will not cool properly, of course, this depends on how much over charging you are talking about. As more refrigerant is added the system's pressures will increase. At some point these pressures will become excessive and this will result in damage to the system usually over time. An A/C system is designed to run on a specific amount of refrigerant. Any deviation from this will cause less cooling ability.