There was this thing that happened a few years ago. Usually the frequency fluctuates a bit, but averages out to 50hz/60hz. Many appliances are using clocks that use the grids frequency to time them. In one instance the grid frequency averaged slightly below their target over a period of time so clicks would show the wrong time (like a minute or two off). They adjusted the grid to be slightly off in the other direction to compensate and make the clocks catch up to real time.

Here’s an article about that: https://www.drax.com/electrification/electricity-causing-clocks-europe-run-slowly/