Corp theft is a daily occurrence in EVE Online. Unlike most MMOs, stealing from other players or organizations in EVE is permissible, assuming that normal in-game methods were employed to pull it off. Simply stated, corp theft in EVE involves becoming a member of the corp and then stealing from the target corporation's hangar(s). Once a thief has access to assets, they can literally clean the corporation out to whatever extent they are able to, depending on their level of granted access. Sometimes corp theft is premeditated, other times it's a crime of opportunity. There are players who approach these heists as a profession, others may do so out of revenge. Regardless of the reasons behind theft in EVE, it's a part of the game and does help establish New Eden's setting as being a harsh, corporation-dominated galaxy where deception is a style of play.
Most corp thefts that this writer is aware of inflict minor damage, often a few billion isk in losses. Generally this isn't enough to cause serious harm to any solid corporation's future, and the only real impact on the corp is emo rage and a few pages of forum drama, and the offender's name being added to Fitz VonHeise's list "The Thieves of EVE" which should be required reading for corp CEOs and Directors who are involved in recruiting players.
But more serious cases involve a thief having Director-level access to corp assets, both the corporation's "physical" items and the corp's funds themselves, and that's when lasting harm can be inflicted. This was the case with a recent theft against EVE corp Destructive Influence (DICE) of the Band of Brothers alliance. Destructive Influence was hit for an estimated 200 billion isk by a (now-former) member of their corporation, "thelung187", as reported by EVE Tribune's Miyamoto Isoruku:
"Beginning a thoroughly trolled thread on COAD, Viper Shizzle of Pandemic Legion announced that thelung187 had stolen 200 billion worth of ISK and property from Destructive Influence, a member of Band of Brothers, dwarfing the infamous GHSC heist. In these days of ever-escalating heists, this one rates only above average -- by comparison, the EVE bank scam netted 671 billion ISK" (Note that the bank scam referred to did not involve "EVE Bank" but rather "EVE Intergalactic Bank" -- the incident is typically referred to as the EIB scam, perpetrated by a player named Cally who also released a video confession.)
While losses on this scale could be devastating for a newer corporation, Destructive Influence has been a part of EVE for a number of years and the theft is not expected to affect them in any drastic way. In addition, EVE Tribune reports that thelung187 didn't take all of the assets he had access to. "Perhaps feeling some residual twinge of conscience, he chose to leave 150 billion ISK in the corp wallet, and their moon mining operations were left untouched. So although DICE is definitely hurting, they still have significant assets in reserve," Isoruku reports.