Has there ever been a case of overvoltage in a power grid?
There are three main effects, 


Blackouts: a total loss of power, generally due to physical damage or a tripped breaker
Brownouts: this is where a substation is overloaded to the point where its output voltage falls due to excessive loading, but not for long enough to trip the breakers
Load spikes: this is where some very large load switches off, so for the small period it takes the power stations to correct for this, the voltage can overshoot the nominal voltage


They are slightly interconnected:


a lot of brownouts imply you're due for a blackout if they don't fix the issue,
a blackout, e.g. an entire suburb losing power all at once, can cause a load spike, and
a brownout is a sign that the load spikes are significant


To actually damage things, generally you're going to need to trim down the margins of your particular device. If there is a lot of grid-tied solar installations in your area this can raise your mains voltage very close to the maximum, meaning any load spikes end up higher in peak voltage, and more likely to damage things. Generally the amplitude of this is limited.

On the other side of the fence, lightning is a very large amount of current. Even the clouds moving overhead can change the relative voltage of your mains ground by a few hundred volts if your country does not require any in-house earth strap/ground rod.