Us really pathetic, pedantic people have already given such a thing an official name: the secondary spoiler.
This can take many forms. For instance the one you mentioned,
Spoiler:where you tell someone there's a surprise waiting for them.
But it can also be telling someone 'wait till you see what happens to Bubba halfway through the movie'. It's not an actual spoiler, but you are indeed chipping away at it. A surprise is that much less surprising if you know it's coming, even if you don't know what it'll be.
Of course, the secondary spoiler is not as unforgivable as a primary one. It also falls more under the category of the rowdy relative that gets too enthusiastic about the movie and is oblivious to any and all tact.
