On August 14, 2003, a massive blackout affected approximately 55 million people in the United States and Canada. The outage was triggered by three tree branches falling on high-voltage power lines in Ohio. This occurred on a hot afternoon when electricity demand was at its highest, causing a power line to sag and disconnect. This initial failure set off a chain reaction, leading to the largest electrical failure in North American history. Within three minutes, 508 production units across 265 power plants were shut down, and the economic cost of the blackout was estimated to be between $6 and $10 billion. The main cause of the blackout was a software defect in the XA/21 system used by GE Energy to monitor the power grid of FirstEnergy, a major utility company. This defect caused the system to fail silently for over an hour, meaning that the gradual collapse of the grid was not detected by operators. An investigation later identified the root of the problem as a "race condition," a type of software error where the outcome of a process depends on the sequence or timing of events. In this case, the error caused the entire alert system to fail at a critical moment. Because the alarm system was not functioning, operators at FirstEnergy did not realize for more than an hour that they were looking at outdated information about the status of their section of the grid. Their screens showed a stable grid, even though it was collapsing line by line just a few kilometers away. Based on this false data, operators made decisions that seemed logical at the time, but they failed to recognize the true severity of the situation. This lack of awareness on the part of FirstEnergy and the regional grid operator, MISO, delayed any effective response to the crisis. While the failure to trim trees near power lines was identified as a secondary factor, it was far less significant than the digital failure that prevented timely action. The investigation confirmed that the blackout could have been prevented with better systems and procedures. In response, FirstEnergy improved its monitoring and tree-trimming practices. The U.S. regulatory body, FERC, also introduced new mandatory standards, including stricter requirements for maintaining power lines near vegetation. Despite these changes, experts say the North American power grid remains vulnerable to similar large-scale outages.