Me, fighting against the internal Amazon culture tooth-and-nail to try to get them to do things that were in the best interests of AWS customers. Some of the most brilliant and customer-centric people I've ever worked with got ran out of there by management (well, one particular engineering director) after 18-24 months.
All the fighting led to them trying to "manage me out" (making my life a living hell, hoping to make me quit), so I decided to stay for a full 4 years in order to vest every ounce of my stock grants, before walking away in 2014 (six months before AWS released "Lambda").
If things are better there now, that's really great for the people who work there. But it was a hell-scape for me to try to bring AWS into the modern age for open-source software in such a backwards-ass internal culture.