A goof in film making is an error made during film production which finds its way into the final released picture. Depending upon the film and the actual scene, the goof may have different effects: a loss in realism, an annoyance, or it could just be funny. It is often a type of continuity error. Goofs are also known as "bloopers" or "mistakes".
There are several types of goofs, for example:
Somebody or something from the film crew is in the picture that wasn't planned (e.g., camera and cameraman is reflected in a mirror).
Chronological or conditional errors (e.g. a cigarette getting longer with the next scene).
Historical inaccuracies and anachronisms (e.g., an HDTV set in a film set in the 1970s).
Geographic: an object or landmark reveals the scene was filmed in a different city than the city it is set. This is very common in Hollywood films that are shot in Canada.
Problems in audio or soundtrack (e.g. a person's lips carry on moving after they have finished speaking).
Intertitles displaying wrong words (e.g. a character called "John" may have his name misspelled Jhon in the subtitles by accident).