In code: Naming things in a manner that makes the code readable. It's a constant challenge.

With cars: troubleshooting and fixing a ferrari 599 without the required factory diagnostic computer. You can't beat a multimeter and some elbow grease. It was a faulty flow meter.

In general: figuring out what to do with my life. It took me a bit but was worth the time. Now I can focus on doing that and just that.