Since the new update, cacti can now grow directly next to any block, but we know they shouldn't and the piece that grows next to the block then breaks off. This has only emerged since the 0.7.0 update that brought tons of bugs that are ruining the game. To view this bug, place a cacti on a sand block, and place any other block next to where the second piece of cacti will grow. Leave for a while and it will grow up instead of breaking. This only works in 0.7.0