According to a couple of sources cited in Wikipedia, Hitler may really have been monorchidic, just like the song says. The Russians allegedly did an autopsy and that was one of the things they claim to have found, or rather, not found. No word on the glandular endowment of the other Third Reich functionaries, however, except that Goebbels, whose wife bore six children, probably was better endowed than the song indicates.
The mouse-in-the-Coke-bottle story is not an urban legend. In fact, mice (and various other vermin and foreign objects) found in Coke bottles have been the subject of numerous for-real court cases, most notably Harris vs. Coca-Cola Bottling Company (35 Ill. App.2d 406, 183 N.E.2d 56, Illinois 1962,) which in turn cites a string of cases going back nearly 40 years involving similar problems. In none of those cases was there any dispute about the foreign object in question actually being in the Coke bottle—only about how it got there, and the appropriate legal consequences of the occurrence. A cursory search of litigation history in Illinois alone reveals more than 25 cases, not counting such unreported cases as the one I handled some years ago involving a used Band-Aid in a bagel. The frequent allusion to such cases as urban legends is undoubtedly part of the vast right-wing conspiracy against tort lawyers. Don’t be fooled, either by the conspiracy or by the Coke bottles. I have not done any research into the incidence of foreign objects and vermin found in Coke cans, however.
Maybe we should start a regular feature column of urban legends that turn out to be real?