Continuity mistake: In the 7th episode of Season 5, the scenes when the "Storm-Trooper" (actually, a robot) passes by The Window do not match. The first time we see it, from the outside, as soon as he passes, Robin notices him and points towards him, but the second time, from the inside, she doesn't point at him.
Continuity mistake: The scenes before Robin and Barney leave the diner vary depending on the angle. From outside we see that Robin fixes Barney's suit before they leave, but from the inside she doesn't.
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 ★