The matchmaker from a previous episode finally has a match for Ted, but he thinks Robin is still the one. Lily has a secret interview for an art school fellowship in California. Barney has a war with an office worker from across the street.
There are certain things in life where you know it's a mistake but you don't really know it's a mistake because the only way to know that it really is a mistake is to make that mistake and go, "Yup, that was a mistake". So really, the bigger mistake would be to not make the mistake because then you'll go about your whole life not knowing whether it was a mistake or not