One compromise I've heard about is to put a vague warning (or no warning, depending on the author's feelings on how spoilery it is) in the standard header, but then provide a more explicit warning - even if it gives away the plot - in either a footnote or another place entirely, and link to it from the header.

Mm. I saw someone try that once, with multiple layers of additional detail. So it was like "Death of a major character" in the header, and you could click on that and you'd get "Daniel Jackson dies" and click on that to get "licked to death by kittens" or whatever it was.

It was a good compromise, but I didn't think it really worked. Those links were just too darn tempting to avoid the spoilers :-)