I first broke a cactus block, it then regrew instantly. Confused, i placed a block next to the second block from the top of the cactus. It the broke the block next to and top of the cactus. But then it started to continuously make break particles and spewing out cactus entities for my inventory. I got over 1.5 stacks of cactus from that one plant. I do believe it is perpetual.

HOW TO DUPLICATE
this may not happen for all since it is a bug
place a block right next to the second cactus block from the bottom without making the marker touch it.
0=block
9=cactus

9
0 0 9
0 9