Director Richard Lester was sued by the actors who claimed they were tricked into thinking the film was to be part of The Three Musketeers (1973). They won their case in court, but did not receive as much money as they would have if they were paid separately for both films.
When Felton buys the knife with which he kills Buckingham, he is charged ten pence. The historical John Felton is indeed recorded as having carried out the assassination with "a tenpenny knife".