Although 'lost' films (films for which no known copy exists, e.g., London After Midnight) are numerous, they are usually not a complete loss since still photographs may yet exist, and often some sequenced continuities survive as well. Not so for lost film sequences, however, as many have been lost forever.
The most famous of all lost film sequences is that of the Spider Pit (or Bug Pit) of 1933's King Kong. As stated in the IMDb: It was a graphic scene following Kong shaking four sailors off the log bridge, causing them to fall into a ravine where they were eaten alive by giant spiders. At the preview screening, audience members screamed, and either left the theater or talked about the grisly sequence throughout the subsequent scenes, disrupting the film. Merian C. Cooper said, "It stopped the picture cold, so the next day back at the studio, I took it out myself." What wasn't stated is that Merian Cooper apparently tossed the cut sequence in the trash since it has not been seen since.
Fortunately, still photographs survived, as did the original script. Filmmaker Peter Jackson, having made his own version of the film in 2005, went so far as to use these surviving materials as guides to recreate what the lost sequence might have been.
Following are YouTube videos of the Spider/Bug Pit sequence, the first being as seen in the 1933 theatrical release, the second being the corresponding sequence from the 2005 remake, and the third being Peter Jackson's imagined recreation. All I can say is that film audiences of 1933 would have keeled over dead had they seen the 2005 version. What do you think?
The Bug Pit Scene (w/o the bugs) from King Kong, 1933.
The Bug Pit Sequence from Peter Jackson's 2005 remake of King Kong. (Graphic)
The Lost Spider Pit Sequence, as recreated by Peter Jackson in the style of the original 1933 film.