Introduction: Blood coagulation tests (BCT) are very important for clinicians to diagnose bleeding or thrombotic disorders and to monitor anticoagulant therapy. Case description: On a Saturday morning, a laboratory technician noted an abrupt rise in the number of coagulation error messages on our ALC TOP analysers. Visual inspection revealed the presence of partially and/or fully clotted citrate tubes and prompted the clinical biologist to further investigate a potential preanalytical cause. Considered causes: Partially or fully clotted blood in citrate tubes can have multiple causes including improper mixing of the tube, under- or overfilling or combining blood samples from different tubes into one citrate tube. What happened: The affected citrate tubes originated mostly from the same clinical departments. Moreover, all the affected tubes shared the same lot number (1 of 7 in use at the time). Visual inspection of 7 unopened boxes of 100 citrate tubes of this lot number revealed one box with nine completely empty and two partially filled tubes and one box with two partially filled tubes. No under-filled tubes were found in the other 5 boxes. Discussion: The blood to additive ratio is crucial for BCT. A sudden rise in clot errors should trigger a thorough investigation to identify the cause. Main lesson: Laboratories should regularly monitor and evaluate the percentage of clotted samples as a quality indicator at scheduled time points. A problem with the volume of additive in citrate tubes should be considered as a possible cause.