Season 3: Episode 7 - Dowisetrepla

Dowisetrepla: Marshall and Lily decide to buy an apartment and proceed to make every mistake in the book. While applying for a loan, Marshall discovers Lily's credit card debt, which leads to a huge fight that Ted is able to recreate, CSI-style, employing his deductive reasoning and clues around the apartment. Barney stumbles upon another use for an open house: it can be a great place to bring a hot, clingy chick you'd never bring to your real home.