Ken Thompson was brought on because he was Ken Thompson, and if you had the means, then not hiring him would be a strange thing to do. The amusing thing about that is that he never jumped through the new hire hoops (a language exam, or something) and therefore could not commit his own code.

https://www.theregister.co.uk/2010/04/21/ken_thompson_take_o... (quote here but it's worth checking out the book)