Speaking of Chicago:
'Suspicious item' shuts Midway Airport for two hours
http://www.suntimes.com/output/news/cst-nws-midway16.html
Apparently the x-ray scanner malfuntioned and brought up a training screen that showed a supposed suitcase with a hand grenade in it. They tracked down the traveller who was going through the security line at the time of the bad scan, examined his luggage, found it all to be unfounded, and reopened the terminal.