Bill Ackman, through his hedge fund Pershing Square, is entangled in a lawsuit with Allergan over potential insider trading related to a hostile takeover bid for the company by Valeant Pharmaceuticals. Allergan alleges that Ackman's substantial stock purchases were made with insider information regarding an impending tender offer, leading to a claim of defrauding stockholders of $1.2 billion. The legal outcome could delay the special shareholder meeting aimed at replacing Allergan's board until the matter is resolved.