So, yes, the error in question was indeed the failure of Spider-Man's spider-sense to warn him of Kraven's drugged dart. As was pointed out in the comments, this is rather ironic considering that of all the characters in Spider-Man's rogues' gallery, Kraven (or rather, all the permutations of Kraven) actually has a way to circumvent the spider-sense. Kraven made use of various potions and concoctions throughout the years to stymie Spider-Man's danger sense, and even after the death of the original Kraven members of his family retained the secrets. In one of the most egregiously bad stories in the history of Spider-Man ("Torment"), Kraven's former lover Calypso used voodoo sorcery to screw with Spider-Man's mind. (DOOM DOOM DOOM DOOM) But Spider-Man was eventually able to rise above it all.*
Anyway - all that would have been necessary to make the incident jibe would be to have Kraven saying something along the lines of - "With my father's formulas, I have recreated the same gas which rendered your vaunted spider-sense useless, in an undetectable odorless form!" It may seem like such a small thing, but they used to give away No-Prizes to explain these kind of discrepancies. The writers and editors back then were no more perfect then they are today, but the illusion of interactivity helped shore up what could otherwise have seemed like a pretty shaky foundation. Now things have changed, and mistakes by necessity go ignored. Things seem brittle to long-time readers, and even pointing out mistakes like these make a fellow seem old-fashioned - recherche, even.
Which brings us to the other error I mentioned. This error has the virtue of being doubly superfluous: you don't need to show Spider-Man's spider-sense reacting to the danger off-panel, because he obviously hears the explosions. The spider-sense isn't just a generic danger detector; otherwise, Spider-Man would be nigh omniscient. The way the spider-sense has traditionally been used, the spider-sense warns Spider-Man of specific, imminent dangers to his person: if someone fires a gun, he can dodge the bullet because he knows about it just soon enough to move in the opposite direction. This is why the spider-sense is useless in close-quarters: he needs to have room to dodge. Similarly, multiple sources of danger make his spider-sense go haywire and result in him not knowing which way to dodge. Nowhere is it indicated that he possesses the ability to know what's going on a couple blocks away, unless it's a guided missile heading straight for him. Since that's not the case, well...
I guess it's just another thing for old-timers to gripe about for no apparent reason.
* If you get that joke, consider your life officially wasted. I know I do.