The greatest hoax perpetrated BY SCIENTISTS has to be Onyate Man. Well, it was more of an April Fool's joke, but it's still great.
"We found a fossil of a hominid, being eaten by an allosaurus dinosaur. Look at the picture." Read More Here
NMSR hung out this hoax hoping that creationists would bite. Interestingly, they didn't. Perhaps they knew that only they are the hoaxers so they are immediately skeptical of any REAL evidence coming from geologists that just might support their case.