MOVIES in the public domain:

The requirement that copyright renewal must occur during the 28th year of the original term – and only during the 28th year of the original term — has caused quite a wealth of copyrighted matter to enter the public domain accidentally or unexpectedly. Although the Copyright Act of 1976 did away with the renewal system, it did not apply retroactively.

For those with an interest in this arcane topic, here is a non-comprehensive list of movies that appear to have entered the public domain.