From Whatis?com (which bills itself as "The leading IT encyclopedia and learning centre")
An ohnosecond is that very short moment in time during which you realize that you have pressed the wrong key and deleted hours, days, or weeks of work.
The fraction of time it takes to realize you've just goofed; for example, right after you hit the send button on an e-mail and realize you forgot to include the attachment. Another great example is that moment of horror when you see the key in the ignition switch just as you're slamming the car door shut. This term was coined by Elizabeth Crowe in her book The Electronic Traveler.
Update: Silly me, google knows all, sees all. Origin of photograph here.