"Now, this isn't definite, but it's just possible that you might find a surprise party when you walk through that door."
Do you all really think the surprise wouldn't be ruined in such a case?
The title *is* a spoiler. Anyone could speculate that a movie might have a surprise ending, just as any person might speculate that they'll find a surprise party waiting for them. If they even thought of such a thing. However, someone mentioning that there might be a surprise party (and calling it an "alert") introduces it into the person's mind, causing them to suspect there is such a thing.