On October 5, 1999 Sprint Corporation and MCI WorldCom announced a $129 billion merger agreement between the two companies. Had the deal been completed, it would have been the largest corporate merger in history, ultimately putting MCI WorldCom ahead of AT&T as the largest communications company in the United States. However, the deal did not go through because of pressure from the US Department of Justice and the European Union on concerns of it creating a monopoly. On July 13, 2000, the boards of directors of both companies acted to terminate the merger. Later that year, MCI WorldCom renamed itself to simply "WorldCom" without Sprint being part of the company.

Edit: Quoted from Wikipedia. Should I look for more instances?