Case #1:  I saw some very strange clouds once.  They were flat, and very black, as black as the text on this screen.  They reminded me of a curtain because the cloud front was perfectly straight.  No rain fell from them.  The weather was perfectly sunny and warm, and then suddently this black curtain sweeps across the sky.  Strangers in the Wal-mart parking lot I was in stopped to comment on its oddness.  As quickly as it came, it swept away.  I didn't have my digicam with me, and I wished I had it at the time.

Case #2:  Clouds raining a foamy substance over a period of two days.  It was summer, and the foam remained on roads, dirt, and in ditches long after the rain quit.