My group has been corrupted by a player who cheats to no end. I wasn't a DM at the time and everyone else was too merciful because he argued relentlessly until he won. So, needless to say, he's on-the-fly interpretation of when the enemy is considered helpless and what powers a skill check grants him, in conjunction with his "ability" to get a natural 20 any time he wanted made it so that most enemies never threatened to kill the party. Once, the DM at the time threw a dragon at us because she was out of challenging ways to fight our party. This guy abuses a series of skill checks to pour glue down his throat and says the dragon chokes to death because it's "real-life mechanics."
After a couple months, I quit that group because I didn't like the guy, even if he played fair he was unbearably obnoxious. When he was eventually kicked out, I came back. However, that's how my players have played D&D for the better part of the last three years, so they don't know how to run from battle anymore. I'm DM now, so I'm having to teach them. I haven't TPK yet, mostly because I'm fudging rolls, but the party seems to expect fatal and near death encounters from me. EVEN STILL, they don't run.