Complications
Sometimes, when a plan goes sour and seems like it can't get any worse... it does. Complications are unexpected problems that compound the difficulty of a challenge. You might trigger a complication by critically failing a check to overcome a challenge, as a consequence of your own decisions, or simply by the GM's design.
Many complications increase enemy Awareness or otherwise make the mission more difficult. A common form of complication is attracting the attention of guards who try to stop, capture, or even kill you. When this happens, the mission may briefly shift into encounter mode as you attempt to defeat your assailants or talk them out of fighting-- hopefully in a way that does not arouse further suspicions. The sounds of battle and argument are loud, after all, so unless the encounter occurs in an isolated area or you have taken special precautions, each round will cause Awareness to rise, at the very least.
Each complication has a trigger that determines when it occurs. It may affect only one of you, or it might affect all party members in a certain area, and it might be something that you can work together to overcome or it might be something that must be overcome individually.
Complications must be overcome before anyone involved may attempt to overcome any other obstacles, and attempting to overcome a complication takes your turn just like trying to overcome an obstacle. Many complications are one-off events that can be overcome automatically, even on a failure, though not without cost.