1 Steward: {rushing into the cockpit} Help! The passengers have been panicking back here! There's been a terrible tragedy! Just after you went into the cockpit!
2 Stewardess: What happened?!
3 Steward: A passenger fell in the panic. Someone tried to catch her but she accidentally had her neck snapped. She's dead!
3 Terry: <gasp!> Who was she?
4 Steward: According to the passenger manifest her name was... Gwen Stacy.

I'm not going to explain this one here. If you get it, then you need no explanation whatsoever.

If you're going, "What? Who's Gwen Stacy??" and you want an explanation, you can find all the information you need at this Wikipedia article, and this one. If you don't get the reference, then those articles are about something you probably don't care about very much. But don't let that put you off. If you really want to understand this comic fully, read them right through.

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2018-09-11 Rerun commentary: Reading both those articles is kind of a spoiler. But if you don't know who Gwen Stacy is, you probably don't care about this particular spoiler.

And it's all been dealt with in a bunch of different ways in the multifarious different continuities that exist now anyway. So it's actually a lot less spoilery now than when this comic was first published.