WHERE TO FIND IT: Yahoo! and more.
CRITIQUE: We here at Cyber Critique HQ have always arched an eyebrow at the fact that, according to Nielsen/NetRatings, some of the highest click-through rates on (non-porn) banners come from ads that look like computer error messages.
Not to mix metaphors, but this strategy sets up a sort of double-edged sword of Damocles. The banners play off a computer user's twitch reflex to click on all Windows-looking error boxes to make them go away. The reflex exists because of the irritating nature of error messages. Then there's the irritation of having not only the faux-error go away, but the page of content the user was viewing go away with it. Who pulled this dirty trick? Well, the click-through gives the answer, and it won't necessarily be a positive branding experience.
The University of Phoenix Online varies this theme to a make a much clearer and more positive statement. Running with a computerlike banner, it shows some of users' favorite keys: enter, control, escape and shift. Each leads to a message about how a graduate degree from its online program will help the user get "ctrl" and "esc" their dreary Dilbert lives by becoming the Dilbert's boss, the Wallys at the top of the executive food chain.
Reinforcing the positives while pointing out the negatives in this bipolar approach should keep that Damoclean sword from dropping.