Whilst building a potato-farm in survival I noticed that the crop in the middle - which is supposed to pop off on any block-update as the light-level is too low - didn't break. No matter how low the light-level is, the crop still stays on its place and can even be bonemealed. Light-updates don't seem to trigger it, nor does the blockupdate from the adjacent pressure plate. When testing in creative and waiting a little the crops díd pop, but incredible slow. It therefore seems to me that the crop only realizes it's too dark when it grows a stage.

This bug makes constructing a villager farm using this crop-in-low-light behavior pretty useless, so I hope it gets fixed soon.