Slapsgiving - S3-E9
Continuity mistake: Marshall turns on the computer, and a readout pops up clearly displaying 2h 45m left on the countdown. He then says "look at that - that means we're in the final hour left of the countdown".
Slapsgiving - S3-E9
Revealing mistake: When Marshall slaps Barney in Slapsgiving you can see that his hand doesn't actually touch Barney's face and there is no audio to match the slap.
Chosen answer: In a flash forward scene in Season 8, it shows him with his first born: a daughter.
XIII