This spring I was working on a library for an algorithm class at uni with some friends and one of the algorithm was extremely slow, we were using Python to study graphs of roads on a map and a medium example took about 6-7h of commission to finish (I never actually waited for so long, so maybe more).
I got so pissed of for that code that I left the lab and went to eat. Once I got back I rewrote just the god-damned data structure we were using and the time got down to 300ms. Milliseconds!

Lessons learned:
- If you're pissed go take a walk and when you'll come back it will be much easier;
- Don't generalize to much a library, the data structure I write before was optimized for a different kind of usage and complete garbage for that last one;
- Never fucking use frozen sets in Python unless you really need them, they're so fricking slow!

That moment when project managers all demand a share of your weekly hours, failing to accept that productivity diminishes exponentially with the number of projects you have to switch between.