We've posted some strange clouds before on Neatorama (like this one). Here's another one: Butterfly Hole punch Cloud, caused by falling ice crystals.
Hole-punch clouds or fall-streak holes, such as the ones shown above near Shreveport, Louisiana, are caused by falling ice crystals. The ice crystals could originate in a higher cloud or be facilitated by a passing airplane exhaust. If the air has just the right temperature and moisture content, the crystals will absorb vapor as they fall and thus continue to grow. Holes are formed when supercooled water droplets in shallow cloud layers freeze (initiated by the falling ice crystals) and release their heat of fusion, which warms the air and evaporates the surrounding cloud. The fibrous, icy wisps falling from the clouds are called fall-streaks (seen above at left in the shape of a butterfly)