What's your most surprising distribution channel, the one that worked when you didn't expect it to?

Spent this week going wider than usual LinkedIn, a few Reddit communities, Indie Hackers, Quora, a republish on Hacker Noon. Expected LinkedIn to be the strongest given the audience overlap. Instead the most interesting thing happened on Indie Hackers: posted something about a real product decision, got a genuinely sharp comment questioning it, tested the question for real, found an actual bug because of it, and fixed it same day. That one thread did more for the product than the "distribution" itself.

Made me realize I'd been thinking about channels purely as reach, when the actual value on a couple of them was closer to free QA from people who cared enough to push back.

What's been the surprising one for you, a channel that turned out to matter for a different reason than you expected?