I abandon probably a good majority of the games I start playing, to be honest.
Most recently was Tom Clancy's Rainbow Six Vegas 2. It's a squad-based tactical-lite FPS, and I was quite enjoying it up until I got to the beginning of Act 6, where you're sent into a mission without any squad. At that point, big problems I had with the game and its predecessor (namely the sheer volume of enemies in levels and how they were often simply spawned in rather than having deliberate placements on the map from the start of the level) basically killed the fun I was having, because without the moderately-competent AI to help me out I was just dying over and over again.
I don't mind a rock-hard FPS, but the Vegas games tried to straddle the line between tactical FPS and CoD FPS too finely, and I don't think the mechanics really meshed together all that well. It's a shame, too, because I quite like the actual gameplay itself and the character progression mechanics they added in to Vegas 2.