What Happens When Smart Curve Fails? How Often Does it Happen?

For Smart Curve to fail you need a MOSFET component to fail. The failure rate of that component is 4.28 failures out of 10^9 devices (3rd party analysis).

In order for you to exceed the maximum string voltage (600V / 1000V) you'll need more than one device to fail (and usually more than that, each failure adds ~5V-10V to accumulated string voltage, depending on ambient temperature). 

The probability of having 2 failures is ~350 times lower than the probability of being struck by lightning in the US during 20 years (assuming 15 modules per string).