The potential for the light leak is with the Sinar Zoom I and perhaps the earlier ones.

One issue is a plastic strip that can deteriorate or go missing. Replace it, and you're good, but people sometimes have problems, because it's disappeared, and they don't know it's supposed to be there.

The other is that the curtains used to adjust the frame size might not close completely or the track could become damaged so they don't stay straight. One of the improvements in the Zoom II, if I understand correctly, is that in addition to the curtain to adjust the frame size, there is a separate conventional darkslide, so the curtains don't have to do the job of keeping the light out when the back is removed from the camera.