  The OPERA collaboration reported [1] a measurement of the neutrino velocity
exceeding the speed of light by 0.025%. For the 730 km distance from CERN in
Geneva to the OPERA experiment an early arrival of the neutrinos of 60.7 ns is
measured with an accuracy of \pm6.9 ns (stat.) and \pm7.4 ns (sys.). A basic
assumption in the analysis is that the proton time structure represents exactly
the time structure of the neutrino flux. In this manuscript, we challenge this
assumption. We identify two main origins of systematic effects: a group delay
due to low pass filters acting on the particular shape of the proton time
distribution and a movement of the proton beam at the target during the leading
and trailing slopes of the spill.
