Deliberate mistake: Perhaps to avoid getting an R rating or parental complaints notice how in the operating room massacre none of the doctors appear to bleed nor do Doc Ock's tentacles have blood on them.
Suggested correction: The problem is in both versions of the film, basically all of the deaths either happen off-camera, or are things like people being shocked to death or killed by being tossed around and presumably having their backs/necks broken. Most of the deaths would be bloodless or near-bloodless anyways, and the camera simply doesn't linger on the bodies enough to show any blood. This is also why the tentacles aren't really bloody. Sure, it was probably ratings-related, but it isn't technically a mistake.
Deliberate mistake: When Spider-Man attempts to rescue Aunt May from Doc Ock, May's umbrella handle latches onto the building's angel sculpture, so the angel is to her right. After Stan Lee's cameo, as May shouts, "Help me!" the watch is on May's right wrist instead of her usual left. When Spider-Man shouts, "I'm coming," next shot of Aunt May shows her hanging from the opposite side of the angel's body. Then Spider-Man shouts, "Hang on," and in the next close-up of Aunt May her watch is on her right wrist again. (00:50:40)
