In over 100,000 miles of sailing, I never saw one.
They happen with some frequency on the SE Coast of the African continent. The Agulhas current
runs South up to 6kts in places. When a slow moving cold front comes through there will be strong (70kt) Southerly winds that appose the current
and the seas can build up to 100'. These seas have been known to produce even higher "Rogue waves".
I saw a ship wrecking yard in the Southern tip of Madagascar
where they piled up about 6 or 8 ships with broken backs. The ship will get on top of one of those monsters and the weight of the ship is only supported in the center of the ship with both ends hanging out in space. The next moment the ship is in the trough with the ends of the ship in the water
and the center hanging in space. This has been known to break the keel
of the ship and actually bend the ship to where it is unusable.
These types of waves are not known in good weather
......the answer is to stay away from areas that they frequent, in bad weather. These days, there is no need to get caught up in this stuff.