Most pissed off? Must have been the time I created a $10,000 hole in a marketing team's budget after trying to build a Tetris-style Facebook app game (back when FB apps were all the rage). It turned out that Tetris, which was up to that point commonly considered as public domain, had been scooped up by a patent troll who had made a convincing case that he had found and gotten agreement from the originator of the idea (a Russian dude who may or may not have been its original source). I had to scrap the whole project and explain what I did to an old-school manager who didn't even know how to operate a computer. That resulted in my losing credibility as a team member for the next 6 years. I never lived it down. I was pissed mostly at myself for following my assumption and not doing any patent research.