I don't know about worst, but the Cypher System (the setting-agnostic version of the system that Numenera runs on) is the only system I've run that the players staged a mutiny over. I know the exact moment they decided to rebel, too:
For the uninitiated, your characters stats in Cypher are a pool of points that you can spend out of to, among other things, reduce the target number of a roll. One of my players was making a really important roll, so she spent like half of her pool to reduce the TN down to a ridiculously low number, then rolled her die and got an 18. Instead of cheering that she made it, she deflated, and went: "Oh. Well I guess I just wasted all those points."
It was such a feelbad moment that I wasn't surprised the next day when they basically all demanded to switch systems to anything else.