I ran into a good heavy fog overhead after a hike along the Pacific coast.  There was no chance for any kind of sunset, but the light was nice and diffuse about an hour before.  The whole scene just screamed of black & white: the sky was grey, the ocean was a palate of silvers.  Everything was set up for mono.
Of all the images I took, this one jumped out at me.  I realize it is hard to see in this small thumbnail, but after shooting the fisherman casting for a while, I re-set myself for a shot of the waves bursting on the cliffs.  And then, completely unplanned, the fisherman walked into my picture and started talking to a swimmer passing by in the opposite direction.  
Nice symmetry and some really nice lighting from the sun breaking through the fog.