Censorship

In their sixth incarnation, the Doctor said that he usually disapproved of censorship. (AUDIO: ...ish)

Behind the scenes
One of the earliest examples of censorship in Doctor Who production history took place during The Keys of Marinus. During filming, actor Peter Stenson tripped on set, and, in his own words, said "a very rude word". They did not reshoot the scene, and simply removed the audio afterwards, to obscure what he'd said.