The potential of health information technology is hampered by new types of errors which impact is not totally assessed. OncoDoc2 is a decision support system designed to support treatment decisions of multidisciplinary meetings (MDMs) for breast cancer patients. We evaluated how the way the system was used had an impact on MDM decision compliance with clinical practice guidelines. We distinguished "correct navigations" (N+), "incorrect navigations" (N-), and "missing navigations" (N0), according to the quality of data entry when using OncoDoc2. We collected 557 MDM decisions from three hospitals of Paris area (France) where OncoDoc2 was routinely used. We observed 33.9% N+, 36.8% N-, and 29.3% N0. The compliance rate was significantly different according to the quality of navigations, 94.2%, 80.0%, and 90.2% for N+, N-, and N0 respectively. Surprinsingly, it was better not to use the system (N0) than to use it improperly (N-).