Since 15w38b, falling blocks will not stop falling if the AAAB below them is greater than one block tall, as a result of:

                    if (BlockFalling.j(this.world.getType(new BlockPosition(this.locX, this.locY - 0.009999999776482582D, this.locZ)))) {
                        this.onGround = false;
                        return;
                    }

.

This for example allows a player to drop sand onto a fence post, where it will be perpetually falling. If they repeat this enough there will be thousand(s) of perpetually falling blocks which will lag both clients and server.