I was at Amzn early '00s when we lost 95% of our market cap. Later at FB I negotiated a down-round in '09, and then in '12 our stock dropped 50% post-IPO. I was on the board of a public company that went bankrupt (Borders) and a start-up that went under (Hello). Some lessons:
If you're the CEO of a fast-growing startup, the mistake you'll make in the next year that you'll regret most will probably be a hiring mistake. Possibly hiring someone established, which you'll feel you should do now you're getting big, and which VCs will strongly encourage.