The accidental subclavian artery puncture is usually obvious. We report a case of unrecognized arterial catheterisation. The catheter had been inserted during anaesthesia after return of dark and non pulsatile blood, and not controlled by a chest radiograph. During surgery, the injection of 40 mL isotonic saline containing 4 g of piperacillin for antibiotic prophylaxis resulted in a transient circulatory collapse associated with ECG tracing of myocardial ischaemia. Postoperative chest radiograph showed that the catheter was in a midsternal position, at the level of the ascending aorta. The intracoronary penetration of piperacillin was considered as the cause for the transient cardiocirculatory changes. The various diagnostic tools of the intra-arterial location of the catheter are discussed. All inadvertent subclavian artery catheterisations published in the literature have been carried out with multi-lumen catheters. The latter can contribute to the failure to recognize the arterial puncture and catheter insertion because of the use of a small bore needle (Seldinger's technique) and infusion with electrical pumps.