I've come across a weird case where this actually matters, so I was wondering if a creature immune to a particular kind of effect is allowed to make a saving throw against it?
The item in question is the Magic-Eating armor quality from MIC. It states that whenever you succeed on a saving throw against a spell that targets you, you heal hit points equal to the spell's level.
Now, let's say this quality is on a Warforged Juggernaut. If a friendly Cleric included him in a Mass Cure Light Wounds spell, would he be able to make his saving throw and gain at least a few health, even if the spell has no effect on him if he fails his saving throw?