Barney Stinson's job has been a mystery throughout the series as whenever the gang asks about it, he only says, "Please", which would turn out to be an acronym for his actual job Provide Legal Exculpation and Sign Everything at AltruCell Corporation and later in Goliath National Bank, meaning he was telling his job all along.
Throughout most of the series, whenever Barney is asked what he did for a living by his friends (such as Marshall in Life Among the Gorillas and Lily in World's Greatest Couple), he dismissively answers "please". This maintains the mystery of Barney's job throughout the series. Many strange events occur, such as Barney stating in The Possimpible that "actually doing things gets you fired", Marshall in The Goat finding a contract detailing the poisoning of the water supply in Lisbon, Portugal in Barney's office, and Barney in Last Cigarette Ever mentioning that the company might murder him one day because of how much he knows, or that he won't be fired but he may end up missing with teeth and fingertips removed in Mosbius Designs. Barney's job is revealed in Unpause, where it is revealed that he has been responding honestly; PLEASE is an acronym ("Provide Legal Exculpation And Sign Everything") and Barney's job is being a legal scapegoat for his company's corrupt activities. He is essentially the company's "fall guy", signing various documents and becoming responsible for any illegal activity. Unbeknownst to the company, Barney is secretly working for the FBI as an informant, with the intention of defeating his boss Greg Fisher, who stole his girlfriend, Shannon, many years ago. This fact explains why Barney never got arrested for many years for signing illegal contracts and papers, as blackmail for keeping his job. A few months after the wedding, he reveals to Greg that he has been colluding with the FBI, informing them of their illegal activities and had Greg arrested, completing his revenge.