This code will print "#<RuntimeError: bork>" in JRuby 1.7.x as well as all MRI versions I tested, but "nil" in JRuby 9.0.0.0:

begin
  begin
    raise 'bork'
  rescue TypeError
  end
ensure
  p $!
end

Removing the inner begin … rescue makes it print "#<RuntimeError: bork>" again. It looks like rescue clears $!. The error matched by the inner rescue doesn't matter, and catching the error and re-raising it also leaves $! unset.