Most of us are familiar with the statements of Chazal that Reuven did not sin (with Bilha) and David did not sin with Bat Sheva, etc. However, what many people do not realize is that there are other opinions in Chazal that hold that these aveiros did happen. Many people do not know that these opinions even exist.
The Gemara in Shabbos 55b starts out with the famous opinion that Reuven did not sin with Bilha but just moved the beds. This is the opinion that everyone quotes. However, the Gemara continues and quotes 2 Tannaim who hold that Reuven did sin. They take the word פחז as an acronym, R' Elazar says Pazasah (you were hasty), Chavtah (you became liable) Zalsah (you disgraced); R. Yehoshua says, Pasatah Al Das (you overstepped the law),Chatasah (you sinned), Zanisah (you had illicit relations).
The Gemara in Kesubos 9a-b discusses the case of David and Batsheva. The gemara there gives 2 answers why she was permitted to him. The second answer is that everyone who went to war in David's army wrote a document of divorce to his wife and therefore she was really divorced. However the gemara has a first answer and that answer is that David did sin and she was permitted to him because it was ones (forced, because she was a minor at the time) therefore she was not prohibited.