Cities are considered as complex systems which consist of highly heterogeneous and interconnected sub-systems and autonomous entities connected by non-linear, multiple interactions. When such a complex system is struck by a hazard, it stops operating as in normal conditions, due to interrupted interconnections among sub-systems. Which sub-systems and/or their interconnections are vulnerable to disasters and may create disruption to the urban system can be assessed by vulnerability analysis. However, how a disruption in a sub-system could affect the urban system cannot be foreseen in every detail prior to the occurrence of an event. In order to further the understanding of cities during disasters, this paper gives examples of consequences of uncertain interactions between urban systems and unforeseen reactions of humans during disasters such as the 1995 Kobe Earthquake, 2005 Rita and Katrina Hurricanes, and the 2010 volcanic eruption of Eyjafjallajokull.