That's actually not far from what happened. Empire Strikes Back was the first time the opening Text Scroll had "Episode" written in it. Hell, George Lucas even went back to change the original to "Episode 4: A New Hope" after Empire came out.
The trailer for that one Terminator movie set in the future spoiled one of the main plot points of the movie The main hero is a special type of Terminator, and the trailer hints to that way too heavily