When a signal isn't properly sync'd -- Usually if one word clock is fighting another. One needs to be the master clock, the rest need to slave to that master sync. Plugging a digital keyboard (or a digital anything) into a digital input of an interface for example --- If the interface is assuming master sync and the keyboard is also assuming master sync, sooner or later those clocks aren't going to be in sync with each other (actually, they're never going to be truly in sync with each other, but they'll be close enough most of the time not to cause seriously audible noise). When they're off by a certain amount (I'm not even positive -- I'd assume a half of a sample) you're going to hear a 'click' of full-band, usually full-scale noise.
You're in a left turn lane in a 2009 Aston Martin -- The guy in front of you is also in a 2009 Aston Martin. You both have your left turn signals on and they're in nearly perfect sync with each other. Yours lights and dims exactly as his lights and dims. But over 15 or 20 seconds, you notice one lagging behind. Over the next 15 or 20 seconds, they're precisely opposite (as yours is lit, his is dimmed). Another bit later, they're right back in sync with each other.
If you could run the signal from his flasher relay to your electrical system, they'd stay in absolutely perfect sync.
You can think of that flasher relay as a word clock -- When you have a chain of digital gear connected together, that one clock needs to keep all the other gear sync'd tother perfectly.