Mirror Image + Displacement

[KarinsDad]

Seems reasonable... But on pg 171 of the PHB, under Spell Failure it says:

If you ever try to cast a spell in coditions where the characteristics of the spell (range, area, or the like) cannot be made to conform, the casting fails and the spell is waisted. For example, if you cast charm person on a dog, the spell fails because a dog is the wrong sort of target for the spell.

If you cast Mass Hold Person, and 1 of those targets does not meet the characteristics of the spell, it fails (unless they can be made to conform to the characteristics of the spell).

I think they were not even considering multiple target spells like Mass Hold Person when they wrote this because it does not explain what happens in multiple target cases.

It is also unreasonable for multiple target spells spells to totally fail because one of the targets is invalid.

It is more reasonable for multiple target spells to fail against the invalid targets and succeed against valid targets. Think of it as invalid targets always have spell immunity against that type of spell, but valid targets do not, even if they are standing in a group of invalid targets.