The most common types of refrigerant contaminants found in HVAC systems include moisture, air, oil, and other refrigerants. Moisture is a particularly common contaminant, as it can enter the system through improper installation, component failure, or poor maintenance practices. Air can also enter the system through leaks or improper charging, while oil can contaminate the refrigerant through worn or damaged components. Other refrigerants can contaminate the system if they are mixed with the original refrigerant, leading to adverse reactions and system failures.