(taken from csh-whynot)

exec 3>&1; grep yyy xxx 2>&1 1>&3 3>&- | sed s/file/foobar/ 1>&2 3>&-
grep: xxx: No such foobar or directory

Normal output would be unaffected. The closes there were in case something really cared about all its FDs. We send stderr to sed, and then put it back out 2.