One that I saw on a startup founders mailing list several years back:
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founder: I have a modest, growing business, but the marketing costs are rather high. What can to do to bring that cost down?
wannabe-founder: whoa, you really failed to validate the core concept and establish your basic value prop. You're going to see an astronomical CAC until you ultimately pivot and achieve LGTM or BBQ. I'd recommend you shut down and find another vertical.