The 2003 US Northeast blackout was caused by the failure of only a few lines that shorted into trees. These line failures created grid instability that resulted, ~5 minutes later, in most of the Northeast losing power in a cascading failure.

Redundant systems have failures in one path quite often that you never know anything about. We get headlines when the failures correlate in the same timeslot.

> one power line failure would never result in this level of catastrophe.

Called N-1 criterion. See https://en.wikipedia.org/wiki/Contingency_(electrical_grid)#...

And it depends. During https://en.wikipedia.org/wiki/2006_European_blackout N-1 criterion was supposed to be holding, in practice not even N-0 was holding and network crashed.

Few years ago nearly entire day European network was sitting on N-0 due to multiple issues in Poland, caused by a heat wave and deeper root causes. There are many power plants and power lines where any further issue would cause Europe-wide blackout.