Flash banners require a small amount of code in them, to handle the flashVar passed to it by the banner admin system. You would be surprised how many inept flash people are out there (on second thought, you probably wouldn't), who forget this small bit of code, and thus rendering the banner completely useless.
A few months ago, I recieved a banner set from a fairly large manufacturer of electronics (lets call them Phony), which were non clickable.
They had to send out new banners... worldwide...
Embarrassing? nooooooo....