Some linac manufacturers provide cones for defining circular electron fields. During commissioning of one of our new radiotherapy units, we noted that the output of these cones has a strong dependence on the diaphragm opening that precedes the cone. In particular, for the 8 MeV beam, the output of a 3 cm diameter cone increased by more than a factor of 2 as the diaphragm was opened from 5 x 5 to 20 x 20 cm. One concludes that the particular mechanical design results in an output factor dependency that is exceptional when compared to the data presented in standard texts such as Khan's "The Physics of Radiation Therapy." The importance of quality assurance and communications with the manufacturer is underscored by this example.