Circumstances outside of the realm of the normal.
In TheCulture, Special Circumstances is the name of a department. It deals with situations where the usual rules of rational, ethical behaviour do not apply in a straightforward way. A bit like the American CIA.
The Culture has very inflexible moral guidelines for the treatment of sentients. When the sentients in question do not share those same high moral values it behoves the Culture to treat them with a certain degree of moral ambiguity. In order that the vast majority of Humans, Drones and Minds not get their fields dirty with such compromises the Culture has a specific section detailed to interaction with outside powers that it dubs Contact. Contact, whilst more morally flexible, has a degree of oversight by the Culture proper - necessary for Contact to carry out its role as ambassadors and diplomats. Contact therefore uses SpecialCircumstances as a combination think tank, intelligence gathering organ and dirty tricks department. SpecialCircumstances is the most morally ambiguous of the Cultures various appendages - in fact it is hard to tell when a Mind has become Eccentric or is just involved in a particularly odd mission for SpecialCircumstances - IainHowe