I started GIMP, the script-fu server, connected via gimp-cl-connect. (- 0.5 0.1) in GIMP's Script-Fu console evaluates to 0,4.0. Odd enough. But in gimp-mode's REPL: Error: (/home/thorwil/.gimp-2.8/emacs/emacs-input.scm : 2) eval: unbound variable: 0,5.0.
I came across this issue, because file-jpeg-save has a few floating point arguments.