We report on a study of field emission accompanied by emission of light in a 20 cell superconducting niobium 3�GHz cavity of the S-DALINAC. The spectral power of the observed light could be interpreted as black body radiation at about 1500�K. The maximum energies of dark current electrons accelerated across the whole cavity were determined from bremsstrahlung spectra and compared with the results obtained by numerical simulations of electron trajectories utilizing a code based on the Leap-Frog method. Explanation of the experimentally observed energies was only possible by assuming a cavity with a strongly deteriorated field profile of the accelerating [pi]-mode caused by detuning of an end cell by 5.4�MHz. The consistency of this hypothesis was experimentally confirmed by comparing the present passband frequencies of this particular cavity with both the calculated ones from a simple lumped circuit model and the eigenfrequencies of other S-DALINAC cavities.