Under SBCL, floating-point numbers were printing incorrectly.
e.g. (format t "~A" 12.34) was printing 12.1234!
The equivalent string literals were ok.

Investigation found that SBCL prints floats as
  (write-string "1234" 0 3)
  (write-char #\.)
  (write-string "1234" 3 nil)
This last call was incorrectly handled by the IF.
I assume it was meant as an optimization...