Continuity mistake: When Ted and Tracey are on top of The Lighthouse, the trees behind them change variety between close ups and the shot zooming out.
Revealing mistake: When Barney and Ted are holding hands on the beach and it starts raining, you can see that the sand is only wet in the place where they are sitting, but dry behind them, and on the left and right side too. (00:16:35)
Revealing mistake: When Ted is on top of The Lighthouse (with both girls), his and the girl's hair is blowing in the wind, but the trees behind them aren't moving at all.
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 ★