Question about air-conditioning cycle All explanations of the air-conditioning cycle are somewhat like this:
https://www.swtc.edu/ag_power/air_conditioning/lecture/basic_cycle.htm
My question is... how do we ensure the refrigerant is in the appropriate state at the appropriate time in the cycle?
For example, what if the high-pressure gas goes through the condenser and it hasn't condensed fully? similarly with all the other stages... what if the flow of the refrigerant is too fast, or too slow...
 A: 
Example Refrigeration System
Referring to the sketch, an electric motor drives a compressor, which takes low temperature, low pressure refrigerant vapor, and compresses it to a high temperature and high pressure.  The high pressure refrigerant vapor goes to a condenser, where it is condensed by a lower temperature environmental fluid (usually air) as heat is rejected to the environment. 
In my example, it is important to send the condensing vapor to the top of the condenser, such that any condensed droplets will drop to the bottom of the condenser, in order to keep condensed liquid from covering up active heat transfer area on the inside of the condenser tubes.  In turn, the condensed liquid goes to the small volume storage tank, where it collects.  From there, the high temperature, high pressure liquid goes through the expansion valve, where its pressure and boiling temperature are reduced.  
Next, the low pressure and low temperature liquid enters the evaporator, where heat is transferred into the evaporator for the purpose of cooling an enclosed space (e.g., the inside of an automobile).  The liquid level in the evaporator is controlled by a level controller, which admits the correct flow rate of refrigerant from the small volume storage tank to hold the desired level in the evaporator. 
The pressure in the evaporator is controlled by a pressure controller on the evaporator’s exit-vapor line, which is also the compressor suction line.  This device controls a valve which “modulates” the pressure of the evaporator, but cannot be fully closed because the compressor should never be isolated from the evaporator.  In the case where the refrigeration system runs low on refrigerant, the pressure in the evaporator exit-vapor line will drop very low, and the pressure switch shown in the drawing will disconnect power to the compressor.
Ameet, regarding your question of how refrigerant is not allowed to accumulate in the wrong places, any real-world refrigeration system will have either electronic or mechanical devices which will prevent such an occurrence.  My drawing, and the details above, give one example of how this can be done.  I am sure that there are several other ways of accomplishing the same thing, and I am also sure that the designers of any real refrigeration system will specify electrical or mechanical devices to deal with these same issues.
Regarding your specific question of failure of the high pressure refrigerant to condense in the condenser, this can indeed happen.  If the person charging the refrigeration system does not pull a hard vacuum on the system before charging with refrigerant, air will be in the system.  This air will tend to accumulate at the condenser, and it acts to impede heat transfer because it is non-condensible.  Until someone purges the air from the condenser by blowing it to the atmosphere, the refrigeration system will operate improperly.
Assuming that there is no air in the system, the original system design takes account of the correct pressures and temperatures needed to condense the refrigerant when using ambient air.  Unless the ambient air temperature substantially exceeds the design ambient air temperature, you can expect the condenser to operate as designed.
Regarding your question about flow rates of refrigerant, my example uses the indicated controllers to ensure the proper flow rate of refrigerant.  Unless a controller fails, or some design criterion is exceeded (either too little or too much of a design variable), flow rates should be whatever is needed for the system to function properly.
