This one is interesting. I wanted to do a very simple RTS-like game for a small LAN party. I knew that we would be sitting at a couple of computers facing a common projection screen. And the whole idea was centered around doing something useful on that screen. So I sat down for a couple of evenings and wrote a simple game where every player gets to be on one of two teams and is given a couple of foot soldiers to command from a top-down perspective. The goal is to eradicate the other team. The big screen would display the remaining strength of each player of each team.
However, when it was time to actually play the game, it failed. The networking code broke down and never before seen gameplay bugs started to appear. I was quite disappointed that evening. But I haven't fixed the bugs since because there was no second opportunity to play that game ever since.