Final answer:
Frank Capra's "It's a Wonderful Life" fell into the public domain in 1974 after a copyright renewal oversight, enabling networks to broadcast it freely.
Step-by-step explanation:
The classic movie that Frank Capra unintentionally allowed to fall into public domain in 1974 is "It's a Wonderful Life." This happened because of a failure to renew the film's copyright.
As a result, television networks were able to air it without paying licensing fees, contributing to its status as a Christmas staple. Directed by Frank Capra and released in 1946, the movie explores themes of despair, community, and redemption through the story of George Bailey, played by James Stewart.