At the core of the issue were plunging oil prices, which ended up having a margin call effect on the bank's swaps exposure; and since Capital One’s waiver lasts until Sept. 30, if energy prices remain low or the bank’s exposure remains above the threshold, it will register as a swap participant or make business adjustments, the CFTC said on Friday.

In short, CapitalOne made a terrible trade, betting via derivatives that oil would not plunge to where it is now — at 17 year lows — and only CFTC intervention prevented a margin call of unknown magnitude from being sent to Capital One's corner office. Which is surprising considering that the bank is a relatively small player in the energy lending and financing business, with energy loans accounting for just 1.4% of its total loan book, according to its filings.