The Will and Grace finale episode had an ad by GE for their "Ecomagination" locomotives. Right before the ad played, something came on the screen about a secret message hidden in the ad and to record it to find it.
They put a one second slide show into the ad that included several slides that you could only see if you watched it by pausing and going frame by frame to read the message and see the images.
www.onesecondtheater.com
Yeah, it was kind of cheesy, but it definitely made me stop and find the "secret message".

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