Charade

Charade is often called the best Hitchcock movie Alfred Hitchcock never actually made. It stars Cary Grant and Audrey Hepburn in a Paris set story that slides between comedy, romance, and genuine thriller tension, with a plot involving stolen money and a rotating cast of suspicious characters that keeps you guessing who to trust until the end.

What makes its public domain status notable is that this is a much glossier, later studio film than most public domain titles. It ended up free due to a clerical error, the copyright notice on early prints was missing the actual word Copyright, just showing the symbol and date, which under the law at the time was enough to invalidate the protection entirely.