Refrigerants should never be intentionally mixed. The most common cross contamination are the two most common refrigerants, R12 and R134a. This is the mixture of the older R12 refrigerant that contains chlorine with R134a. Chlorine was found to cause damage to the earths ozone layer and has been replaced with the newer and environmentally safer R134a.
They are in research for an improvement as R134a itself is found to contribute to global warming. These refrigerants must be contained as it is unlawful and unethical to release them into the atmosphere. Today's manufacturers use R134a as their preferred refrigerant.
These two should not be mixed in an AC system because they are two different compounds with different temperature to pressure characteristics. The mixture is called azeotrope. Though the low temperature to pressure characteristics of azeotrope may be close to 134a, the high temperature high pressure characteristics vary considerably. The symptoms of this blend result in high system and head pressures. High head pressure will cause the compressor and other system components to fail prematurely.
Use a refrigerant identifier or refrigerant gas analyzer any time a refrigerant mixture is suspected. These devices typically detect R12, R134a, R22, air, and hydrocarbons. Hydrocarbon gases like butane and isobutane often set an audible alarm and may be explosive and hazardous to the technician or occupants of the vehicle. It will display the amounts of each type of refrigerant as a percentage of the total blend.
An added advantage of using one of these tools is that it will also inform the technician of the amount of air in the system. The average shop recycling machine is incapable separating R12 and R134a. There is a special procedure and equipment for handling and separating these mixed refrigerants.
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