Manufacturing defect: every manufacturing process isn’t 100% free of errors, even more so if humans are involved in producing the unit. There are multiple joints and welds which could cause a leak if they’re not made with high precision. If there are manufacturing defects within the AC, leakage will occur during the initial stages of use and thus are often repaired under warranty. Incorrect installation: vital a neighborhood of AC installation includes making flare nut connections and bending the copper pipes, which ironically are the foremost common areas of a refrigerant leak. To avoid this, flare nuts should be sealed with Teflon tape, and copper pipes should only be bent via a tube bender (copper pipes are soft, and easily develop sharp bends if the force applied thereon is concentrated during a little area). Regular wear and tear: Air conditioners are pressurized units. With time the bolts and joints can become weak and develop a leak.