Marvel fans around the world are well-aware of Spider-Man's propensity to be bogged down by his own guilt. Peter Parker is defined by guilt as much as Batman is defined by vengeance: it is a trait that both characters can never escape, no matter how hard writer after writer tries. But in Giant-Size Astonishing X-Men #1, Spider-Man's guilt actually managed to save the day rather than ruin it - while Reed Richards of the Fantastic Four used his magnificent intelligence to solve the same problem (and failed).
Peter Parker's self-loathing and constant guilty conscience first appeared at the shocking conclusion of Amazing Fantasy #15, in which Peter discovers Uncle Ben's killer was in fact a burglar whom he failed to stop earlier that day. Thus began a lost history of Spider-Man blaming himself for the many tragedies in his life - only some of which are his fault. The Night Gwen Stacy Died is perhaps one of them; Spider-Man's method of catching Gwen Stacy actually caused her death (in fairness, she would have died regardless of his actions). Aunt May's almost-death in Spider-Man: One More Day is in a moral grey area, and something that Spider-Man fans still argue about to this day.
In Giant-Size Astonishing X-Men #1, written by Joss Whedon, a bullet from the mysterious Breakworld has been fired and immediately endangers the Earth. The Fantastic Four, X-Men, Avengers and other heroes know the bullet is coming and also know the round is shielded by a mysterious "magical protection." The protection comes in the form of mind-manipulation: every character with a plan to stop the bullet believes they stopped it before it actually arrives. Reed Richards engineers a miraculous contraption that actually works - or so he thinks.
Spider-Man believes he stopped the bullet with a massive web - but then his ever-present guilt gets the better of him. While every other superhero is confident in their abilities (especially Reed Richards), Peter Parker always doubts himself, and this doubt snapped him out of the magical protection long enough for him to 'wake up' the other heroes. "You saved the world, right?" he says to Storm, before gesturing at a dozen assembled heroes, staring straight into space, their minds clouded. "So did they."
Eventually, Kitty Pryde manages to phase the bullet through the entire Earth - but only after nearly every superhero on the planet failed. Reed Richard's confidence turned out to be his weakness, while Spider-Man's doubt and guilt managed to save himself. The self-proclaimed smartest man in the world, Reed Richards fell into a pit of his own self-assuredness - but Spider-Man being constantly aware of his own failings is what makes him human.