Admit it, we all make extensive amounts of mistakes in games the first time we play them. New Vegas is a testament to this. So post some of the stupid mistakes you made during your first playthrough.

Killing Boone because I did his quest wrong and failed.

Trying to kill Ranger Jackson because the Pip-Boy mentioned "get rid of" in the Heartaches by the Number quest.

Trying to take a shortcut to Vegas by going North of Goodsprings.

And then, in my most recent, all-knowing playthrough: Refusing to explore Zion. I missed a bunch of stuff.