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.