When arrows are shot into some blocks (Eg. (Grown) Wheat blocks, Grown Melon\Pumpkin Stem and maybe even more), the arrow's inGround NBT tag does not update. It's possible that this is caused by the plant's hitboxes.

Steps to reproduce:

1. Create SethBling's explosive projectiles device (Found here: http://www.youtube.com/watch?v=vxDi41dmgMc)

2. Fire arrow into a Wheat block, or into a Pumpkin\Melon stem (however, this has to be straight down). You should see that the arrow will not explode.

OR

1. Repeat Step 2.

2. Open the world in MCEdit (with the NBTEdit filter)\NBTEditor and you should see that the arrow's inGround tag is not set to true\1

I'm pretty sure it is not updating as the explosive projectile system detects for arrows with the inGround tag set to true. As the arrow does not explode, it's likely it's not updating.