I would have to say my silliest mistake as a nooblet would have been on nancy. We relocated to one of the hives, but I was the only one there, my other team having ramboed off and died somehow.
So, I jump in the chair being the all knowing nooblet that I figured I was, and drop 2 IPs. Then I can't figure out how to get out of the CC. I eventually ended up freaking out, with the whole team yelling at me, and I just turned my computer off mid-game, still in the chair. Took a few days before I played again... Good times, eh?