One of the things I'm most proud of as a dev is a tiny contribution to the Nim standard library. It's a single commit and very simple, but made the PostgreSQL adapter consistent with the other RBDMSs' adapters. At the time, I'd been using Nim for only a couple of days, but that didn't stop me from being able to contribute in a meaningful way.
I'm amused and humbled that despite all of the work I've done over the past decade, it's quite likely that a seven-line change that I did mostly in ignorance will be the thing that impacts the most people going forward.
Here's the change, if you're curious: https://github.com/nim-lang/Nim/pull/6381/commits/921cc53e40...