The chopping of a cactus by placing sand on the side of it on normally drops the whole cactus. When there is a hole dug next to the cactus for the falling sand to fall into, the bottom two blocks of the cactus do not get chopped.

Steps to reproduce desired behavior:
1a. Plant a 3 block high cactus in the center of a 3x3 patch of flat sand.
2a. Place sand on the side of the 3rd block of the cactus from the ground.
3a. Cactus drops 3 blocks.

Steps to reproduce issue:
1b. Plant a 3 block high cactus in the center of a 3x3 patch of flat sand.
2b. Remove one of the blocks along the perimeter of the 3x3 patch of flat sand.
3b. Place sand on the side of the cactus directly above the hole created by step 2.
4b. Cactus drops 1 block. The two bottom blocks of the cactus are not dropped.