In a comment thread, a question came up regarding immunity and saving throws. Specifically, does immunity mean automatically succeeding a saving throw?

I suspect it does not. Conceptually, a saving throw is an effort or chance to resist an effect while immunity is not being able to be affected.

In practice, it's more important as immunity is a pretty general rule, and if it means automatically succeed at a save, then there are more specific rules that cause auto-failures that would undermine immunity. E.g. tripping a stunned water elemental.

Is there are reason that compels you to play D&D 5e where immunity is an automatically successful saving throw, and do cases where more specific automatic saving throw failure effects come up?

@Someone_Evil I've reviewed those. The problem of saying it's a saving throw that automatically succeeds is conditions like stunned are more specific rules that would make creatures immune to prone auto-fail the strength saving throw against a trip.

Along that vein of investigation, I found a nice answer regarding choosing to fail a save. I liked it because it went through save by save and illustrated which are conscious contests a creature could choose not to participate in.