Example?
In the 1980s, someone tried to patent the idea of swapping bytes to convert data between systems that used different byte orders. This had been done since the 1960s, and engineers considered it so obvious that nobody had thought to patent it. The US Patent Office was actually going to award the patent, and considerable pressure was applied by the software engineering community before they finally disallowed it.
At a later date, a patent was actually awarded retroactively for the invention of the semiconductor. The inventor had contributed nothing to the actual development of semiconductors, but managed to convince the patent office that he had developed the fundamental priciples first. The actual inventors didn't have documentation that went back that far. This was a blatent attempt to use the patent system to rob all semiconductor developers. I don't know how this one ended up, but I know that it cost a great deal of money for many businesses which had done real development work.