I remember many years ago, when I was an apprentice, one of the skilled men was knocking out little black boxes for a fiver. He claimed they reduced your bills to nearly nothing.
Basically he was stripping the primary winding off a transformer and then bringing 2 leads off the secondary. Apparently you then shoved these 2 leads up into your meter, (not sure if it was the supply or load side), and it slowed the spinning disc down. A week or two before your meter was due to be read you removed the device.
One day he was very late coming in to work. When asked why he said his meter was due for reading in about 2 weeks so he was taking his device off. When he turned round there were two blokes from MANWEB standing there, (his meter was on an outside wall at the side of his house), and they had been watching what he had done. Turned out he had slowed his down so much he had actually had it running in reverse when they caught him.
Ended up with a conviction for theft of electricity, a huge fine and a suspended sentence. He was also ordered to back back a couple of thousand £'s as compensation to MANWEB.
After that a number of these devices started to appear on his workbench. People who had bought them didn't want to be caught using them.
And no, I never bought one. I was still living with my parents and there was no way dad would have allowed such a thing in the house. He found 2 beer glasses from the social club in the house one day and escorted my eldest brother whilst he took them back!