sed -e 's/foo/bar/; t; s/bar/foo/'

sh-3.2$ printf 'foonbarn' | sed -e 's/foo/bar/; t; s/bar/foo/'
sed: 2: "s/foo/bar/; t; s/bar/foo/
": undefined label '; s/bar/foo/'

[2addr]t [label]
         Branch to the ``:'' function bearing the label if any substitutions
         have been made since the most recent reading of an input line or
         execution of a ``t'' function.  If no label is specified, branch to
         the end of the script.