The flexible sections fail at the crimped fittings, and the hard lines fail where they rub against the body.
Listings state which section of the circuit the line serves: discharge, suction, liquid or the line to the rear evaporator on vehicles with rear air. Getting that wrong means a line that physically fits but connects the wrong ends of the circuit.
O-rings and sealing washers are consumables and are usually not included. Green HNBR O-rings are used for R-134a and different material is used for R-1234yf; using the wrong one produces a leak within weeks.
Any open line means the system has been exposed to moisture. The receiver-drier has to be replaced whenever the circuit is opened for more than a short time.
