Piltdown Man is the premier scientific hoax. In 1912, a skull was unearthed at Piltdown in England. It appeared to be that of a creature somewhere between an ape and a man and got called Piltdown Man. It was really a composite - a hoax. The skull was actually that of a prehistoric human but the jaw bone was from a modern orangutan. The bones had been treated to make them look old. Remember that the techniques we have for dating such relics did not exist in 1912. Forty-one years later, in 1953, a team at the British Museum performed rigorous tests and exposed the hoax.
For decades many people have speculated on who might have done it. There is a large list of suspects, including Sir Arthur Conan Doyle. No one knows for sure, but there are some things one can conclude. First, the orangutan jawbone was treated to make it look older. This required someone with sufficient knowledge to realize it was necessary and to do it. Second, the fake skull had to be found if the hoax was to work at all. This suggests that the hoaxer was familiar with the digging being done at Piltdown. They either buried it where they knew the investigators would dig, or were able to subtly influence the dig sites to ensure that the fake would be discovered. These suggest (but don't prove) that it was an inside job.
The following web sites will provide excellent background and descriptions of this historic hoax.