You think it's going to take 2 weeks, but really it's going to take 3 months. And those 3 months are going to feel like an eternity. Don't be this guy.
Learn more about Underestimating the Complexity of the Task at Hand
You think you've thought of everything, but nope. The unexpected is always lurking, waiting to ruin your day.
Learn more about Not Accounting for the Unexpected
You think you're good to go, but really you're just winging it. And by "winging it", I mean you're probably going to crash and burn.
Learn more about Not Having a Plan B (or C, or D...)