We aim to characterize the current status of knowledge on the accuracy of open-cluster parameters such as the age, reddening and distance. These astrophysical quantities are often used to study the global characteristics of the Milky Way down to very local stellar phenomena. In general, the errors of these quantities are neglected or set to some kind of heuristic standard value. We attempt to give some realistic estimates for the accuracy of available cluster parameters by using the independent derived values published in the literature. In total, 6437 individual estimates for 395 open clusters were used in our statistical analysis. We discuss the error sources depending on theoretical as well as observational methods and compare our results with those parameters listed in the widely used catalogue by Dias et al. (2002). In addition, we establish a list of 72 open clusters with the most accurate known parameters which should serve as a standard table in the future for testing isochrones and stellar models.