Revealing mistake: When Tigress is hatching a plan to escape Gongmen Tower after they have been trapped, she looks out over the wall. But take a close look at the archers: they all move in perfect sync, indicating the same animation models were used for all of them.
Continuity mistake: When Po and the Furious Five are in the top floor in Shen's palace, the lights above them are brightly lit. After Mantis smashes the cannon through the floor, the lights turn off.
Chosen answer: No, it is merely a joke about the size of Mantis and his surprising fighting prowess. "Fear the..." is a somewhat common joke. For example, the University of Maryland uses the unofficial slogan "Fear the turtle!"; the Milwaukee Bucks use the slogan "Fear the deer", etc.
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