Ken Thompson gave a famous speech upon receiving an award titled "Reflections on Trusting Trust" In the speech, he described inserting an almost impossible to detect trojan horse in the C compiler and the login command.
Although the paper is written as if the whole thing was a gedanken experiment, there is little doubt (I think) that Ken really implemented the artifact. In his own words n that paper: "I would like to present to you the cutest program I ever wrote".
Moreover, there are a lot of sources and folklore in the Internet suggesting that the thing was not only written, but also distributed. Is there any statement made by the author explicitly denying (or accepting!) that?