From BlackberryToday:

The Wall Street Journal is reporting that the BlackBerry outage we all felt earlier this week was caused by a non-critical system upgrade and routine that had not been tested enough. This caused a chain reaction that stopped e-mails from flowing to many businesses, governments, and individuals, according to the The New York Times.

Research In Motion (RIM), of course, did not think the software would cause a problem like this, as it was intended to improve the system. The Times says RIM reacted quickly and tried to switch service to its backup system, which had been tested and worked in the past. Unfortunately, this time the backup system did not work, delaying service even more.