First job while in college... Was working for web dev team lamp set up before lamp was lamp (year was 2000).

Had deadline one week after summer vacation. Worked non stop a couple of days to get shit done and didn't make it. Got in a conflict with my manager in front of the team and I blew my steam off. Quit on the spot.

Lessons learned:

1. Don't be a fucking idiot when estimating work.
2. Be cool with other teammates, nobody cares about drama and nobody has to feel sorry for you.
3. Uhm, plan? Had entire fucking vacation to get work done. I was a fucking moron.
4. Burning out is stupid and unproductive.
5. Your manager can be as poor in management as you are. Your job is to try to make them better at it, as they have less visibility in the details.

Next job in grad school. Worked for a security company. Direct manager had the bright idea to make execs sign the change requests. WTF. Code was in Perl/php, a mess. Team rewrote back end DB access , taking over six months, or more, failing twice the deadline. After a final 48 hour burn out, we ship and get laid off the week after.

Lessons learned:

1. Don't work for dicks.
2. Don't be a dick yourself.
3. Don't work for dicks.

Third job was in silicon valley. It was a great company, and I stayed there for five years.