In 1998, Kevin Smtih began his acclaimed run on Daredevil, with Joe Quesada on art, which helped launch the Marvel Knights line of comics (Marvel Knights was actually a separate branch of Marvel, with a separate editor in chief and everything! It was made up of the infrastructure of Joe Quesada's own company, Event Comics). The main plot of Smith's storyline was that an old Spider-Man villain, Mysterio, was dying, and decided to go out in a bang by thoroughly destroying his arch-nemesis, Spider-Man. However, at the time, Spider-Man was currently a different character (Ben Reilly), so Mysterio decided to adapt his plan to someone else, namely Daredevil.
At the end of the story, Mysterio kills himself.
The problem, continuity-wise, came when a month later, Mysterio appeared in the pages of Amazing Spider-Man...fighting Spider-Man!!
It all came down to the fact that, when Spider-Man editor Ralph Macchio was asked if they could use Mysterio, no one mentioned that they were going to kill him off! The Spider-Man editor already had plans to use Mysterio in the pages of Amazing Spider-Man a few months after the Daredevil story was to finish.
As you may recall, Daredevil ran a bit late, so the conclusion of the arc ended up coming out AFTER the Amazing Spider-Man arc had begun, so while Mysterio was dying in one comic, he was the big villain in another.