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If you live in a city with tall buildings, take a look at the night skyline. The buildings probably have flashing lights on them to help aircraft pilots to identify the shapes of buildings and not fly into them.
You might notice that the buildings flash in synchrony, even though they are hundreds of metres apart. What's going on?
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The idea is that all the lights on a given building should flash together. That way, an aviator can visualize the solid space between lights that blink the same way. Blink same? Don't fly. Blink different? That's a gap.
And that's how it worked for a while. A building's owner would wire all the lights together and feed them with a centralized oscillator signal.
Later on, when wire got expensive and buildings got taller, they switched to short-range radio, with one transmitter sending the blink signal, and each light having a receiver in it.
This worked fine, because all oscillators will drift eventually, so every building gets a different blinking pattern.
Then GPS happened.
Now you can buy a beacon like this one, which is standalone and has a GPS receiver in it. Anyone who's cobbled together an NTP server will know that the GPS system can provide microsecond-accurate time as a side-effect.
So these beacons don't care where they are, but they all know exactly when "now" is. And they can synchronize to each other by synchronizing to the GPS signal. It makes maintenance _so_ easy.
Trouble is, these beacons come preset to a particular blink pattern, and when you buy one (ten, fifty) you're probably not going to bother to change the blink pattern in them all. What if you forgot to do one?
So what happens is that your building is synchronized to GPS, and so is your neighbour's building, and so is _their_ neighbour's building. The whole city flashes once every two seconds, like clockwork.
Completely negating the whole point of having one blink-pattern per building, and rendering aviation around tall buildings unsafe again.
It's an accidental own-goal, brought about by the efficiencies of global, perfect, timekeeping.
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@futzle Huh. I was under the impression that the synchronisation was deliberate to not create visual cacophony by randomly blinking lights. The documents I have available about the rules for equipping wind turbines also say that the lights should be synchronized. But that might be a German speciality? On the other hand, the rules for aviation safety ate pretty international... 
@ascii158 From the replies I’ve had, it does vary somewhat by jurisdiction. One document I found about Australian CASA rules mentions simultaneous flashing; you can find it somewhere in the replies.