The following program does not give the same result with -O2 and without:

import qualified Data.ByteString as B

main = let a = B.singleton 65 in print (func a a)

func y z = B.append r s
  where
    r = B.map (succ) x
    s = B.map (succ . succ) x
    x = B.append y z

Result observed with GHC 7.6.1 (bytestring-0.10.0.0):

$ ghc --make test -fforce-recomp && ./test
[1 of 1] Compiling Main             ( test.hs, test.o )
Linking test ...
"BBCC"
$ ghc --make test -fforce-recomp -O2 && ./test
[1 of 1] Compiling Main             ( test.hs, test.o )
Linking test ...
"CCCC"