I can reproduce a bug in the pretty printer.
$ ecl -norc -shell t.lisp
prints for me (ECL from git, debian, amd64).
(LET ((#:TARGET-STRING1137 "aa"))
#:G0)))(DECLARE (IGNORE #:G0)))(DECLARE (IGNORE #:G0)))))
ie. the last line is badly indented and has the wrong characters.
Shortening the "vooo" line, so that the line with "#:g0" goes into the same line, makes
the bug disappear; so it seems to be something in the vicinity of (maybe-output),
(really-start-logical-block) or something like that.