What's the smallest signal that told you your product was actually working?

Dashboards are loud. The signals that matter are usually quiet, and I'd like to collect yours.

I've written here before about how our most active users at Murror turned out not to be our happiest ones. That changed how I read numbers. Sessions, streaks and retention curves all measure that people came back. None of them tell you whether coming back was good for them.

Since then I've been more interested in small, quiet signals that are hard to put on a chart. Things like a user who stops using your product because they no longer need it. Someone who writes in to correct a detail, which means they read closely. A person who recommends it with a caveat, because they're being honest about who it is and isn't for.

My own rule of thumb is that a signal is worth trusting when it would still look good if you were wrong about your product. A big engagement number can rise because people love something, or because it nags them. A quiet signal like "they told a friend, unprompted" is harder to fake, even by accident.

I don't think there's a clean answer here, and I suspect the best ones differ a lot by product type. A dev tool, a marketplace and an emotional wellbeing app should not share the same definition of "working".

So I'd like to ask: what was the smallest thing that made you think "okay, this is actually working"? And the reverse: what metric looked great for a long time before you realised it was telling you the wrong thing?

Not looking for polished case studies. A one-line answer is just as useful. I'll read everything and reply to as many as I can.