According to the New York Times, a pump on the recently installed toilet in the American laboratory, Destiny, broke on Sunday and contaminated part of the system.
Designed to inject chemicals that help separate liquids from solid waste, the pump was a pretty vital piece of the toilet's operation and it being out of commission meant the the seven astronauts from the recently docked Endeavour, were forced to use the toilet on the shuttle. The six members of the ISS crew used a second toilet in the Russian module.
This isn't the first time the toilet on board the space station has been out of order. In May of last year, the toilet on board began to malfunction with the liquid waste disposal part of the unit only working intermittently. A new toilet was delivered to the space station last year and has the added functionality of turning urine into drinkable water.