Waterberry.

Waterberry sort of sounds like a quaint hamlet in a fairy tale of old, but it's not.  No, waterberry is a grapevine disorder that interrupts the development of ripening berries.  Waterberry is known to manifest itself in two distinct ways; one occurs on the very tip of the rachis (which I've observed over the years in Vinoland's Cabernet sauvignon); the other, as photographed in the Pinot grigio above, can impact berries anywhere throughout the cluster.  The affected berries become flaccid, shrivel and eventually turn raisin-like.  Certain grape varieties are more susceptible to the disorder than others.
What causes waterberry?  Hmm.  Studies have shown that there is no clear relationship between the disorder and irrigation practices, although heat stress is thought to be one likely cause.  It is possible that waterberry occurs when grapevines are overcropped, giving rise to competition in the vines for a limited amount of the nutrients and materials needed for both fruit and tissue development.  Possibly the xylem vessels in the pedicels become plugged up with tyloses (tyloses are outgrowths of parenchyma cells: parenchyma is soft cellular tissue) thus obstructing the movement of goodies to the berries.  It also has been noted that in growing operations were girdling is practiced there seems to be elevated instances of the disorder.  Who knows for sure?  I don't.   
It's not like I see this phenomenon every growing season.  In fact, I had to actively seek out a Pinot grigio cluster with waterberry damage for this post.  Growing conditions are different each year, one vintage is not like the next.  And the relatively small occurrences of waterberry in Vinoland's grapevines do not negatively impact the overall crop.