everyone says launch day is chaos, how chaotic are we talking

We're preparing for our first PH launch and we think we've checked everything: page, visuals, first comment, demo, team briefed.

But "we think" is doing a lot of work in that sentence 😅

I know this is probably the most asked question here, but I'm genuinely curious: those of you who've launched before, what's the one thing I should absolutely know before launch day?