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.