Valid and invalid concelebration

Here’s a quick liturgy question. If two priests are concelebrating Mass and the principal concelebrant messes with the words of consecration to such an extent as to render the Mass invalid normally, but the co-celebrant uses the correct formulation, is the Mass still invalid? And what does the person in the pew do if he’s sure of what the principal celebrant said, but not sure what the co-celebrant said?

This is theoretical, but relevant I think.