When vanity metrics start giving you the wrong impression, by boosting your unfounded confidence for example, you’re going to clash with reality sooner or later. This could happen after you placed a bet that seemed reasonable, but in reality it was too risky or even unfeasible, catching you off-guard.
This could be because you’re focussing on the wrong things, for example page views vs signed up for a trail, thinking you’re in a much better spot than in reality.
On the contrary, if your metric isn’t performing well, it can ruin your confidence.