Security issues for the power industry have become increasingly relevant during the past decade as the industry has relied more and more on communication protocols. The Generic Object Oriented Substation Events (GOOSE) protocol is defined in IEC 61850 for the purpose of distributing event data across entire substation networks. In this paper we demonstrate a practical attack by exploiting weaknesses in GOOSE. We also show that this attack can have devastating consequences on the reliability of the grid and is capable of creating a widespread interruption in power generation and distribution.