In version 1.5.2 and before, it was possible to harvest crops planted on farmland (wheat, carrots, potatoes, etc.) by blocking all light sources. This would cause all of the crops to pop off at once as soon as any one of them received an update. In 1.6.1 and 1.6.2 pre-release, they pop off one at a time.

A crop popping off no longer triggers its neighbors to update and pop as well. I do not believe this is/should be the intended behavior because the crops still pop off with a redstone update. Upon testing with a BUD, it appears that the crops popping off due to insufficient light does not create an update at all.

http://youtu.be/4NyD0rDYhuE