Security consultant Bruce Schneier writes about the Boston police's embarassment over their over-reaction to a simple, legal, advertising stunt: a cartoon network hired a guerrilla marketing company to place cartoon characters made from blinking lights around ten major US cities. While the police of the other nine cities recognised them for what they were, the Boston police panicked -- and having belatedly realised what a stupid over-reaction that was, decided to dig themselves into a deeper hole by arresting the artist and trying to bluff their way out by claiming that the signs were bomb hoaxes.
What isn't funny is now the Boston government is trying to prosecute the artist and the network instead of owning up to their own stupidity. The police now claim that they were "hoax" explosive devices. I don't think you can claim they are hoax explosive devices unless they were intended to look like explosive devices, which merely a cursory look at any of them shows that they weren't.
Scheier's blog has many more details.