It may well be a particular feature of your camera and its firmware (both not exactly young and typical), so you'll probably have to investigate on your own. As a brief historical background (far as my ailing memory serves), the problem of camera hanging on distance override surfaced some time in the beginning of the year, when the then active CHDK maintainers were trying to unify the code. It was found, in particular, that some (mostly A series I believe) cameras hung on calling MoveFocusLensToDistance, if not in manual focus mode. It later turned out that (most) Ixus'es somehow responded to MoveFocusLensToDistance without hanging, even though they lack manual focus mode. Although in some of them the mentioned f-s MFOn,Off appeared to be present in the firmware, the issue was not investigated any further, at least not in the mainstream.
Now as an example, in Ixus950 (my cam, perhaps one of the last VxWorks, already with propset 2), MoveFocusLensToDistance moves the lens correctly (unless it is retracted or in play mode), but a prior call to MFOn "notifies" the firmware about the manual mode to the extend that it even writes "manual focus" tag to EXIF.
