The grid is never exactly 50Hz, it varies quite significantly. On the long run, the grid operators try to average to 50Hz as close as possible, just so that clocks continue to operate somewhat accurate.

In fact, it is even in the FAQ of the article:

"The transmission grids of the countries of Continental Europe are electrically tied together to operate synchronously at the frequency of approximately 50 Hz"

Based on the article the control limits are 49.8 to 50.2 Hz (+-0.2hz). That's less than 1%, not very significant.

One of the places I lived in Austin, TX had a range with a clock that used grid frequencies (60hz in the US). It was dead accurate. I only changed it for daylight savings, it never need adjustment.

The US used to guarantee 5,184,000 cycles per day. That’s not, like, 1% or anything comparable, that’s zero (with temporary deviations). Any clock which referenced line frequency would be dead accurate over the long-term. There’s a system called Time Error Correction (TEC) which would do this (refer to WEQ-006 and the like). Obviously this error is measured relative to some reference clock.

I’m not sure what the current status is, but my understanding is that there are efforts to retire this system and allow the speed to drift more.

Oh found it. Looks like this went on for more than a couple minutes:

https://www.theguardian.com/world/2018/mar/08/european-clock...