Each call to either gmp-drawable-get-pixel or gmp-drawable-set-pixel leaks 
about 256 bytes of memory (with an RGB image).

How to reproduce:

Create an empty RGB 256x256 image and execute the following script:

(define (script-fu-memory-leak-bug img drawable)
  (let*
    ( (i 0)
      (pix (gimp-drawable-get-pixel drawable 50 50))
      (a1 (car pix))
      (a2 (cadr pix))
    )
    (while (< i 1024)
      (gimp-drawable-set-pixel drawable 50 50 a1 a2)
      (set! i (+ i 1))
    )
  )
)

The script-fu process will allocate (and never free) about 256K each time 
the script is invoked. Changing set-pixel to get-pixel (and removing a1 
and a2) produces the same result. Tested in 1.2.3 for Linux and 1.2.4-pre 
for Windows; the 256 byte estimation has only been checked with the 
Windows version.