When

/execute @a ~ ~ ~ /clone ~ 0 ~ ~ 124 ~  ~ 128 ~

is typed into a commandblock, it fails to execute the clone, but when it, or one of the 2 parts is put in the chatbox, it works just fine.
it also works just fine in either with both parts separately.
I.E.

/execute @a ~ ~ ~ /say hi

or with

/clone ~ 0 ~ ~ 124 ~  ~ 128 ~