  Tolerance against failures and errors is an important feature of many complex
networked systems [1,2]. It has been shown that a class of inhomogeneously
wired networks called scale-free[1,3] networks can be surprisingly robust to
failures, suggesting that socially self-organized systems such as the
World-Wide Web, the Internet, and other kinds of social networks [4] may have
significant tolerance against failures by virtue of their scale-free degree
distribution. I show that this finding only holds on the assumption that the
diffusion process supported by the network is a simple one, requiring only a
single contact in order for transmission to be successful.
