Except where they contradict one another.
This is absurd.
An amendment, by definition, is an alteration. It is, if you will, a changing of the mind--or, in the case of a constitution, of the expressed will of the people. If an amendment runs counter to a previously articulated statement within a constitution, the inevitable construction is that the amendment removes/alters that statement, so that the application of the law going forward conforms to the amendment and not the original un-amended language of the constitution.
For this reason, it is structurally impossible for an amendment to violate anything within a constitution. It cannot be done, not under any circumstances. A constitutional amendment may be morally wrong or suspect, but it cannot ever be "unconstitutional." An unconstitutional constitutional amendment is a contradiction in terms.
Thus, the only challenges to amendments are procedural ones. California's Constitution is somewhat unusual in that it draws a distinction between amendments and revision, placing a requirement of legislature involvement for a revision. The court rulings I have referenced earlier in this thread (
Livermore, Amador, and
Mcfadden) establish the distinction as being one of scope, determined by several factors including the length of the amendatory/revisory language, the number of sections of the constitution impacted by the language, and the significance of the impact of the language.
What is most important to realize about procedural challenges is that they are intrinsically arbitrary; there is no principle of common law one can draw upon to ascertain what "should" be the just outcome--indeed, there is no "just" outcome at all, but merely an affirmation of whether the established procedures have been followed or no. In the matter of Proposition 8, the only justiciable question before the court was whether the language was revisory in nature, in which case the matter would have been improperly presented to the electorate and would of necessity be invalidated on basis. Once the Court held the language was not revisory in nature, the potential for overturning the electoral outcome was nil.
One can plausibly argue that justice and fairness call for validation of gay marriage. One cannot plausibly argue that Proposition 8 and its rejection of gay marriage is unconstitutional.