It seems Uniscribe(Microsoft OpenType Engine) has some restrictions which disallow some invalid combinations (ref:http://www.microsoft.com/typography/otfntdev/arabicot/other.htm).
I will request experts to please discuss whether these type of checks (Grammar or Spell) should be made at Uniscribe level? My personal opinion it should not.
--
Best Regards,
Atif Gulzar
I ◘◘◘◘ Unicode, ɹɐzlnƃ ɟıʇɐ