I'm not sure about all the particulars, but I was surprised once on Mason Lake late in the afternoon with long period swells when it was otherwise dead calm.  Scared the crap out of me and my passenger! 

It was so calm that the water was a mirror of the low scattered clouds and I had no idea how high I was above the water.  I had just reduced my descent rate but not quite enough and the last few feet I got the wave reflection out of the corner of my eye and fortunately we skipped off the top of a swell  I had just enough power on to pull and add a bit more to skip off the top of the next swell before we were airborne again.

I shallowed the descent rate and was able to make the landing.  Once down and having gathered my wits, I was amazed at the size of the swells that we just could not see. Had we had a shallower descent rate it may not have been such a big surprise but that wave action out the corner of your eye is a real eerie feeling.