Continuity mistake: Robin says she never played any team sports in 1-6 "Slutty Pumpkin". But in this episode she talks about her relationship with her father, being on a hockey team that won a championship and she made out with her teammate.
Revealing mistake: In the front shot of Ted in front of Stella's apartment, it is painfully obvious that he is in front of a green screen and not actually on a street.
Other mistake: At the end, on the steps in front of Stella's apartment, Stella's daughter stares at the camera for half the shot.
Answer: After running the marathon for Marshall with no training, he's allowed to ride the subway for free. While sat on the train, the muscles in his legs start to seize up after the marathon to the point it is painful to move his legs. This is why he can't move them. It's the same reason your body aches after a good session at the gym the day before.
Ssiscool ★