According to some of the recent posts I read, no, there is no such thing as a pH crash... it is a myth.
What happens is a person goes to their tank and finds dead fish. They test their pH and find it's super low. So they say they had a pH crash because their KH was too low. But all that's happened is a CO2 overdose. That's what killed the fish. Had their been a higher KH, the pH wouldn't have been super low, but the fish still would have died.
That's the theory anyways.