There was a link at the bottom of the article to another blurb about the "mistake" - this seems to be a common problem with computer users:
"There is a high probability that the trader, on entering the erroneous trade, ignored an error message flashing on the computer screen. Traders who use the same computer system were quick to defend any such action, however. They said that on an average day similar messages “pop up about a half dozen times and are ignored”."
Wow, it's amazing their systems aren't clogged with junk - "Do you want to download and install this ActiveX autodialer control?" comes to mind...
Beyond that, I'm with Joe - a program should do some math to look before it leaps.