In the late 1990s a famous artist brought his "revolutionary fuel saving" device to the university mechanical engineering department I worked at for independent testing. It turns out he was tuning for idling. So his car engine used very little fuel while sitting at the lights doing nothing and produced crap performance and crap fuel economy while actually moving the vehicle. There was a lot of that going on.
However there have been a small number of real advances from non-experts, I think it was sometime in the 1950s that somebody thought of running a fuel pump in the opposite direction to the normal gravity fed tradition and it made a difference. Normally it's someone who improves one thing and doesn't understand that it doesn't help the entire system at all.
A classic that wasn't actually from the layman was the all-ceramic engine. The idea was that you could run it really hot and get more out if the fuel. Fantastic performance on a testbed, but the extra mass of the more involved cooling meant that one you tried to move it around it performed worse than what it was supposed to replace. Whoops. They only thought of a part and not the implications to how it was actually going to be used, just like that artist who was paranoid about his "invention" being suppressed.