A45

The only such situation I know of is when you are held in the mouth of a creature with the Snatch feat when that creature uses a breath weapon that allows a reflex save.

Losing dex to AC has no effect on your saves at all. You can even make a reflex save against a Fireball while sleeping, though your dexterity would be considered 0 and thus give a -5 penalty until you woke up.