The purpose of Phase I designs is to estimate the MTD (maximum tolerated dose, in practice a dose with some given acceptable rate of toxicity) while, at the same time, minimizing the number of patients treated at doses too far removed from the MTD. Our purpose here is to investigate the sensitivity of conclusions from dose-finding designs to recording or observation errors. Certain toxicities may go undetected and, conversely, certain non-toxicities may be incorrectly recorded as dose-limiting toxicities. Recording inaccuracies would be expected to have an influence on final and within trial recommendations and, in this paper, we study in greater depth this question. We focus, in particular on three designs used currently; the standard '3+3' design, the grouped up-and-down design [M. Gezmu, N. Flournoy, Group up-and-down designs for dose finding. Journal of Statistical Planning and Inference 2006; 136 (6): 1749-1764.] and the continual reassessment method (CRM, [J. O'Quigley, M. Pepe, L. Fisher, Continual reassessment method: a practical design for phase 1 clinical trials in cancer. Biometrics 1990; 46 (1): 33-48.]). A non-toxicity incorrectly recorded as a toxicity (error of first kind) has a greater influence in general than the converse (error of second kind). These results are illustrated via figures which suggest that the standard '3+3' design in particular is sensitive to errors of the second kind. Such errors can have a very important impact on drug development in that, if carried through to the Phase 2 and Phase 3 studies, we can significantly increase the probability of failure to detect efficacy as a result of having delivered an inadequate dose.