What is the problem if your air conditioning unit continously gives cool air until it freezes?

You have probably overfiled the AC, whether it's F12 or 134A. The low pressure sensor has to detect a depletion of refrigerant on the low side befoe the AC compressor can cycle off. If there is too much refrigerant the sensor will never detect a low pressure condition and the AC compressor will never cycle, it will just stay on and freeze all condensate. Is the air cold or just cool? AC systems no longer use R-12 and certainly do not use 134A, unless it is in your vehicle. If the air is just cool and your pipes are freezing, you have a system low on refrigerant, R-22 or 410. You could also have a dirty filter or coil. Either way, you should have your system checked. Question should be more specific as to application. However here is another possibility. If this is a home a/c system then the contactor (relay) on the outdoor unit (condenser) is stuck or energized by a control circuit. This allows refrigerant to be pumped throughout the coil while the blower is off. This will also cause the coil to freeze up. Also if the thermostat is not operating correctly, it does not 'call' for the blower to come on then the same thing could happen. I would strongly suggest calling a professional.