What was the one leak you fixed that changed your results?

Everyone talks about strategy, but most improvements come from fixing leaks. For me, it wasn't learning fancy GTO concepts, it was simply folding more marginal hands out of position.
What was the biggest leak you fixed?
I'd love to hear stories.

For me, it was learning to respect river aggression. I used to talk myself into calling down on the river with medium-strength hands thinking, "they could be bluffing." Once I accepted that river bets and raises at micro/small stakes are almost always pure value, folding those marginal hands saved me endless buy-ins.