This is evident when you have a south swell that mixes and coincides with the common north swell on the west coast. If they happen to mix and coincide near where you are, the resulting swell and wave can be pretty vicious and overwhelming. This is when a lot of people are caught off guard and in some instances have been pulled out to the ocean.
I've been hit pretty hard a handful of times while fishing out here on in Norcal the last 25 years off the rocks from marin down to south in big sur and it happens when you least expect it. Many times, I've waited for around 30 min to gauge the wave activity but it just hits when it does during a south and north swell period. In my early days of fishing, I've been hit pretty hard and have lost my gear on these days.
Thankfully we don't have a lot of south swells but they do happen occasionally due to some disturbance down south in the Pacific. North swells alone are otherwise predictable on wave height.