S90 users, did you notice that firmware bug ?!
When I shoot at ISO1600 there is a strong yellow-orange cast that does not appears at other ISO !
But at other high ISO, some yellow shadows appears sometimes.
Attachments are ISO800, 1000, 1250, 1600, 2000, 2500, 3200.
Have a Nice Day
Is it possible that ISO 1600 is producing a shutter speed that happens to hit a sour spot with the artificial lighting in the room in which you took the picture? Getting inconsistent white balance problems would tell me you are seeing the effects of the natural frequency shifts of certain types of artificial lights.
Perhaps try shooting outdoors in natural light and see if you can repeat the phenomenon.