Robust yet Fragile Systems? 

Basically, a system needs some degree of complexity (design) to achieve robustness, but it does so at the cost of increasing sensitivity to (hopefully) rare events

However, while a system's robustness is bounded, its complexity is not. 
This gives rise to a robustness-complexity distribution is heavy-tailed (we will see a picture of this on the next slide). 
For example, if you build your network such that when you add a router, that router is more likely to be directly connected to a well connected router, the distribution of the number of circuits per router will be heavy-tailed [LUN2004].
Current POP design practices produce just this effect