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 earth’s ozone layer, and has been replaced with the newer and environmentally safer R134a.
These two should not be mixed in a system, because they are two different compounds with different temperature to pressure characteristics. It's called called azeotrope. Though the low temperature to pressure characteristics of azeotrope may be close to R134a, the high temperature, high pressure characteristics vary considerably. The result of this blend is high system and head pressures. High head pressure will cause the compressor and other system components to fail prematurely.
A refrigerant identifier or refrigerant gas analyzer is used when 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 can be explosive and hazardous to the technician or occupants of the vehicle. It displays the amount of each type of refrigerant as a percentage of the total blend. It also displays of the amount of air in the system. The average shop recycling machine is incapable of separating R12 and R134a. There are special procedures and equipment used for handling and separating this mixture of refrigerants.