There are a few contenders for this accolade - Credit Suisse's 2000 acquistion of DLJ, Royal Bank of Scotland's 2007 ABN AMRO deal. But this has to take the biscuit.
It's Bank of America's 2008 acquistion of Countrywide.
And The Wall Street Journal has been tallying up the costs:
Mortgage-security repurchases - at least $13bn
Mortgage bond settlement - $8.5bn
Loan modifications - $8.4bn
Settlement of class action - $600m
Mortgage abuse settlement - $335m
Federal Trade Commission claim - $108m
Losses - Bank of America's mortgage business (some of which is Countrywide) lost $8.9bn in 2010 and $3.8bn in 2009.
Nice one, Ken!