It was a software update that caused the problem.

I worked for years as a computer systems administrator. I ALWAYS do software updates during non-working hours. What I like to do, if I can, is come in on a Saturday morning, which gives me all weekend if necessary. If I can't, then I do the work at night. In any event, I give myself enough time to test everything, including connectivity, before the start of business.

Also, the problem at United Airlines was that a router -- a machine which lets computers talk to other computers -- had gone down and there wasn't a backup. One would think there would be backup routers, so if one failed, they could immediately go to another one. A high-end Cisco router, a 12816/GSR has a list price of about $75K. For a company like United, that's peanuts.