I was trying to harvest full-grown Nether Wart, but the game was acting as if there were nothing there. Then I noticed the block outline was only 1/2 block high. If I aim lower, the Nether Wart breaks, but if I aim high the game doesn't see me make contact with it.

The attached screen shot shows the block outline covering only the lower half of a mature Nether Wart.

There were many previous reports of Nether Wart textures being out of sync with its growth stages, including that fully grown Nether Wart had the same texture as newly planted ones. I suspect that when that bug was fixed, the developer didn't think to check whether the model needed correction as well.