Bankers colluding to keep the LIBOR rates fixed at preferential values.

The LIBOR rate is in theory merely an indicator of the average of interest rates estimated by the leading banks in London, that they would charge to borrow from other banks. In practice, this rate has significant impacts downstream, since it affects many other loan / savings accounts / etc. rates. So the fact that this rate was manipulated, not a true reflection, ended up being A Very Big Deal. An article about it from the Economist: http://www.economist.com/node/21558281