The Wizard of Oz
Dorothy is always wearing her famous ruby red slippers in the land of Oz... except for this one shot. For whatever reason, Dorothy is seen wearing a pair of brown Mary Janes. The scene is so subtle and barely noticable upon first watch. Judy Garland but have changed out of the shoes in between takes and forgotten to put them back on. Whoops!
Raiders of the Lost Ark
One of these things is not like the other... In this scene in Raiders of the Lost Ark, someone dressed in modern-day clothes can be seen walking in the background. Obviously, this does not fit the time period. A production member must not have heard that they were recording and walked right through the background... And then, the editors left it in! How did nobody notice this mistake?