well, you are right I have misused the word culprit and call (probably part of the misunderstanding ). still my original thought was that you could find useful to know that among the hundreds of involved parameters it was the mask_s and mask_t values to be trashed on MacOSX after a couple of frames (respectively one between format/size/en_tlut/tlut_type), e.g. so that you add the fatalerrors suggested by Arbee only to the routines that can affect those quantities and not everywhere.
but apparently my idea was naive and it is not so useful as I hoped. fair enough.