Prompted by an actual case of potentially life-threatening infusion pump malfunction, we investigated the effects of wire breakage(s) within the syringe size sensor circuit in a Graseby 3400 infusion pump. The circuit wires within the sensor were systematically broken. The syringe sizes recognised by the sabotaged circuit and the actual sizes of syringes inserted into the pump were compared. Thirty-eight per cent of the possible wire breakages resulted in a smaller syringe size being recognized, causing the infusion rate to be too fast, and 38% of the possiblewire breakage resulted in a larger syringe size being recognized, causing the infusion rate to be too slow. The volume delivered for each different size of Terumo syringe as a function of distance travelled by the plunger was measured. The errors ranged from 0.4 to 2.6 times that of the expected rate. Only 1.3% of the possible wire breakage(s) were recognised as errors by the pump. The infusion rates were not affected in 22.5% of the cases. Wire breakage within the syringe size sensor in infusion pumps is yet another potential source of infusion error, with important safety implications.