A blooper, also known as an outtake, is a moment in the filming or recording of a piece of media, in which a mistake occurs that ruins a take. Such common mistakes to occur in these circumstances range from actors forgetting or stumbling over lines, breaking or falling out of character (known in the theatre business as "corpsing"), issues with props, or unintended intervention from the production crew. General instances of cast and crew messing around on set is also a common feature.
These productions typically make no secret of these failed takes, and it is common for a "blooper reel" to be put together for home video releases, as many of these errors result in humorous situations. Blooper reels are, indeed, a reoccurring bonus feature on home video releases of Doctor Who media.
History
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For modern Doctor Who, blooper reels were included on the DVD boxset releases of Series 2, Series 3, and Series 5.
Torchwood had blooper reels released for its first two series, released on their respective DVD boxsets.
The first and fourth series' DVD sets for The Sarah Jane Adventures also had blooper reels.
A single blooper from Face the Raven was uploaded to the official Doctor Who YouTube channel on 22 November 2015.