After the release of Amazing Spider-Man #500 many fans were worried about the inaccuracies, especially the way the death of Gwen Stacy played out. You can check out some of the discussion here at the Spider-Man Message board. For those who are too lazy, here's how it played out: Someone points out:
"I don't know if this has already been mentioned but in the bridge scene in #500 Gwen Stacy was conscious screaming for help at Spidey - but in #121 wasn't she unconscious? I remember reading an editorial or something where Ralph Machio was saying it was a possibility that she may have been dead before the fall, which we all know is not the case."
Then someone else points out:
"Yes, but following the Schrodinger's Cat paradox example from last issue, the Cat being used in physics to explain the possibilities of several alternate dimensions, Peter was actually vising alterate realities during this issue. So the Gwen on that bridge was conscious. He might have visited any one of an infinite number of Gwen's on any infinite number of bridges, but she was conscious on that one. She is unconscious on many others. So it wasn't a mistake at all and can easily be explained away.
And that goes to show you that it was just an alternate future Pete who went down by a law enforcement officers gun, not the real Peter, also."
And then this reply shows another inconsistency:
"I agree, these were all alternate realities that he visited. For instance, in the Vulture scene, Spidey crashes through the watertower. In ASM 2, Vulture dumped him in the water tower through the opening. But, that means that all of regular reality that we've been following for years is now dead, and Spidey exists in a reality that is a few minutes behind the one that ended in ASM 499, doesn't it?"
And finally John Romita Jr. shows up and says:
"I don't know if you all noticed but none of the re-run scenes was entirely a perfect cover of the original scenes. It was JMS' thought pattern, correct me if I'm wrong Joe, that the scenes we chose were the same scenes but not the exact second depicted in the original classics. So the scenes we did could be seconds or minutes before or after. Did that make any sense? It sounded good in my head but by the time it got to my fingertips...I don't know!"
The thread is larger, but this is the essence. All the flashbacks were inconsistent, and all the inconsistencies were apparently intended by JMS.