Florida Keys Electric Cooperative Power Outage: Oct. 11, 2004

• 
  Length of outage: Approx. 1 hour
  
  
• Number of people affected: Unknown -- most residents of Florida Keys

Here is a relatively rare instance of a hardware upgrade causing an outage. The unique geography and climate of the Florida Keys was clearly a factor.
Read the original article HERE.
Here, I think it best to quote directly from the article to give you a sense of what happened:

One strand of a corroded shield wire unraveled during its removal from service today, causing a power outage from Islamorada to Key West. Florida Keys Electric Cooperative was pulling the wire for replacement when one of its seven twisted strands failed.

The broken strand swung into the energized transmission lines below it, causing a short in the transmission line. The shorted line caused a power outage beginning at 12:40 p.m. The outage began south of Snake Creek Bridge at mile marker 86.

The strand of shield wire failed over water while being pulled along Long Key Channel, complicating correction of the problem.

As a little background, the "shield wire" is the un-insulated wire that runs from pole to pole above the wires that carry the actual current. It is intended to reduce service interruptions and equipment damage by intercepting lightning strikes. In a salt-air environment such as one finds along coastlines, these conductors tend to corrode fairly quickly.