Tying my identity to the company I worked for. Tech-corporate culture incentivizes you strongly couple your sense of self with your company, which was frankly disastrous for me and for anyone else I've known to fall into the trap.

I could see my company struggle and fall behind, and I desperately wanted to help turn things around (after all, this company was part of who I was). I overextended myself, made bad engineering and interpersonal decisions, and burned out. Worse, I stayed at the company for far too long, thinking that I could really make a difference in the trajectory.

Reading the article about "keeping your identity small" helped me greatly in not making the same mistakes.