At this point, it's almost as big of a cliche as the cliche it's making fun of: Whenever someone says, "I want everyone to give 110%!" someone else responds with, "You can't give more than 100%. It's the maximum amount possible." Cue people smirking at the stupid guy who delivered such a tired and lame motivational gimmick, or at least that must be what people do because jokes about mathematical fallacies rarely bring in the real yuks.
Except that it is entirely possible to give 110%. If I work at a call center, and the goal for the week is to take seventy calls a day, and I take seventy-seven, I have given 110% of that goal. If I sell vacuum cleaners, and my average sales-per-week is 200 vacuum cleaners, and this week I sell 220, I have given 110% of my usual effort. If I am a running back, and my personal best is a 100-yard rushing game, and I have a 110-yard rushing game, I will have given--guess what? That's right, 110% of my personal best.
Numbers greater than 100% exist. They are used routinely in mathematics and everyday life. When you tell someone "You can't give more than 100%," all you are really telling them is, "I'm not only a smug and arrogant cynic, I'm also bad at math, too!" Which isn't exactly the message you want to be sending. (I hope.)