I remember once in the 1970s seeing a clock run backwards!

A friend burst out laughing when I pointed it out to him.
It was an electric clock running directly off the mains and it was very late at night. The only thing I can think of is that the phase of the mains supply was somehow inverted? I've heard of the mains supply being speeded up overnight to compensate for power cuts but never actually reversed, besides which there hadn't been a recent power cut. The next morning the clock was showing the correct time again.