In this paper, the impact of HARQ signaling errors caused by unreliable feedback on the throughput of communication systems like UMTS LTE is investigated. We derive analytical expressions for the calculation of the average number of transmissions per data frame as well as the system throughput based on a general feedback channel model. In combination with the residual frame error rates (FERs) after channel decoding, measured for a system with error-free feedback, a semi-analytical evaluation of the influence of unreliable feedback information on the system throughput can be carried out for any feedback channel condition. Consequently, time-consuming simulations of the respective system with unreliable feedback can be avoided. The theoretical results are verified by extensive simulations of the UMTS LTE physical layer. They are also applicable to other communication systems.