The Python 2 to Python 3 migration has been incredibly painful, and there are still a huge number of people on Python 2. Ditto for Ruby 1.8 to 1.9. Javascript has had a nightmarish compatibility and upgrade story.
Scheme and Erlang are fairly rigidly specified. Clojure is young enough that breakage is treated as expected between versions.
Lisp is a whole family of languages - but if you mean Common Lisp, it's been extensively tied to an essentially fixed spec for a very, very long time.
for(split(" ","tsuJ rehtonA lreP rekcaH")){print reverse . " "}print "\b.\n";
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