A pretty forgettable episode of How I Met Your Mother, since there was literally nothing new about it. I'm not necessarily against familiarity in comedy, sometimes the same joke gets funnier the more you do it, but that wasn't the case here.
We've already seen Marshall and Robin bond, and they do hang out together, at The Hoser Hut. Sure, there they're probably ensconced in the safe topics of cold weather, sports, and cold weather sports, but still, it seems dangerously close to a continuity error.
Usually, Barney's fake histories fall into the "funny no matter how often they do it" category, but The Mermaid Theory was off just a little bit, as even the character seemed to acknowledge. "It's a thing," he insists, a little too sheepishly.
I'm sure many people were intrigued by the narrator's unreliability. Here Future Ted conflates personalities, timelines and arguments, only to reveal that he's getting ahead of himself, and that his Barney-Lily story takes place much later, and ends with Ted wearing a dress, somehow. That bears a pretty strong resemblance to the whole goat at Ted's 30th, no 31st, birthday trick.
Not even Robin in a manatee costume could save this episode.