Continuity mistake: In the car, just after Barney discovers they are not headed for Atlantic City, the DVD screen that has been hanging down from the car roof is suddenly stowed away between shots.
Continuity mistake: When Lily's mother is looking at the gift Lily has just opened, the blue bow on the gift vanishes and then reappears between shots.
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 ★