4 command blocks, Above, below, north, and south of a restone block
South: /execute @a ~ ~ ~ /fill ~ ~-8 ~ ~ ~-5 ~ sponge 0 replace redstone_block 1
North: /execute @a ~ ~ ~ /fill ~ ~-3 ~ ~ ~-3 ~ redstone_block 1 replace sponge
Above: /setblock ~ ~-1 ~ redstone_block 1 replace
Below: /setblock ~ ~1 ~ air 1 replace

the above and below act as a clock, and the other two should replace a sponge block two blocks under the players feet with a redstone block with a damage value of one, and replace only redstone blocks with a damage value of one with sponge 4-7 blocks below the player.

Instead the sponge is replaced by a redstone block with no damage value, making the southern command block not work.

This worked in 14w25b