Imagine, in JPMorgan Chase's case, the frustration at being the leading credit derivatives house, high-yield-bond underwriter and fixed-income research firm (among other things) and then losing on some bets on interest rates.
As you know, before the collapse of the mortgage-backed bond market in 2007, banks routinely offered mortgages worth more than 90% of the value of a house - and even 100% mortgages were relatively common.