Wien had required that Heisenberg did a "Praktikum" (basically a practice in physics experiments), but there was some equipment lacking and Heisenberg wasn't interested enough to find out where to get it. He thus turned towards other things without looking much into the experiments he was supposed to do, for example measuring the splitting of spectral lines by help of an interferometer. Then came the day of the oral exam:
"Wien was annoyed when he learned in the examination that Heisenberg had done so little in the experimental exercise given to him. He than began to ask him questions to gauge his familiarity with the experimental setup; for instance, he wanted to know what the resolving power of the Fabry-Perot interferometer was... Wien had expained all this in one of his lectures on optics; besides, Heisenberg was supposed to study it anyway... But he had not done so and now tried to figure it out unsuccessfully in the short time available during the examination. Wien... asked about the resolving power of a microscope; Heisenberg did not know that either. Wien questioned him about the resolving power of telecopes, which [Heisenberg] also did not know."
(From Jagdish Mehra, Helmut Rechenberg: "The Historical Development of Quantum Theory Vol. 2 - The Discovery of Quantum Mechanics 1925" p. 67)
Wien wanted to fail Heisenberg, but Sommerfeld, in whose exam on theoretical physics Heisenberg had excelled, put in a strong word for Heisenberg. Heisenberg passed the doctoral examination with the lowest possible grade. Many years later Heisenberg would recall
"So one might even assume, that in the work on the gamma-ray microscope and the uncertainty relation I used the knowledge which I had acquired by this poor examination."