Parts of Germany, France, Belgium, Spain, Portugal, Croatia and Italy were blacked out on 5 November 2006 when German power controllers switched off a cable that left some areas lacking power and others overloaded.
Good article about it on BBC website
It is interesting that inter-connecting national grids intends to secure supply. However, it adds complexity which can contribute to failures. This seems to be the result of new technology. The likelihood of failures are reduced but there consequences when they happen are often much greater.