During axillary brachial plexus block for hand surgery, the axillary artery was accidentally punctured. After skin disinfection of the operation site a livid discoloration of the hand appeared. The initial intention of stopping surgery and performing an angiography for clarification of the suspicion of a vessel lesion was dismissed after recording the pulse at the wrist and all fingertips employing a pulsoximeter. Further investigation showed that the livid discoloration of the hand was a product of the interaction of the octenidin solution used for pre-operative hand disinfection with the polyvidone-iodine solution used for surgical skin disinfection. This case report shows that interactions of topically administered pharmaceuticals have to be taken into consideration. Lack of knowledge might lead to unnecessary and unjustified diagnostic procedures which imply additional costs and dangers for the patient.