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.
How I Met Everyone Else - S3-E5
Other mistake: In the scene where Barney is describing The Vicky Mendoza Diagonal of The Hot-Crazy Scale, his words describe her getting hotter, (i.e. lost ten pounds) not less crazy. On the diagram shown, she goes down in craziness, not up in hotness.
How I Met Everyone Else - S3-E5
Continuity mistake: During the flashback in which Barney recounts the story of how he and Ted met (which was previously shown in the pilot episode of the series), there is a green circular plaque (that reads "Winner of the Queen Cup") on the wall in the background which wasn't present in the pilot episode. Also, Barney's left arm is in different positions than it was in the pilot.
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 ★