I have a scenario that will likely never happen, but I am curious in how it would work.
First, the preliminary, Combining Magical Effects:
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.
Now the setup:
Bob the 3nd-level Generic Wizard and Doug the 3rd-level Earth Wizard face off. Bob knows Doug’s favorite tactic and readies a spell. Doug casts Earth Tremor under Bob. The spell say, "You cause a tremor in the ground within range. Each creature other than you in that area must make a Dexterity saving throw."
But Bob was ready and casts Earth Tremor as a 2nd-level spell for his reaction making it more potent (more damage) in the same area.
So what happens?
Doug’s casting means Doug is not targeted by the tremor. But as a reaction Bob casts a more powerful version where Bob is not targeted by the spell. So does that overpower Doug’s spell? And if so, does that mean Bob no longer has to make a Dexterity save and Doug does?
There may be other spells that do this but Earth Tremor was the first I found with wording stating that regardless of the target point, the caster is not affected.