How I Met Everyone Else - S3-E5
Factual error: When Ted is talking about what Marshall told him after he met Lilly, the two boys are playing with Playstation 2 controllers. The scene takes place in 1996, and Playstation 2 didn't come out until 2000.
Factual error: When Barney puts the CD in Marshal's laptop it's upside down but works. Barney points the label which has writing on it towards the camera as he puts it in the laptop.
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 ★