If the priest changes the words of consecration, the mass is invalid. Or if the priest is an impostor. I don’t think there’s much else. It’s good to ask questions but don’t worry too much. I’ve been a Catholic for 64 years and have only ever been to one mass I knew was invalid because the priest had dementia and skipped part or all of the consecration. The bishop finally took action, thank God.
The other two things would be if he is not using bread and wine, or if he does not intend to perform the sacrament (for instance, because he is giving a demonstration).
It is important, though, to point out that the questions of a “valid Mass” and a “valid Eucharist” are not the same thing. In this regard, think about Canon 927 of the Code of Canon Law:
It is absolutely forbidden, even in extreme urgent necessity, to consecrate one matter without the other or even
both outside the eucharistic celebration.
Theoretically – this is what’s being forbidden here – a priest could consecrate the Eucharist outside of Mass, just by saying the words of consecration over bread and wine with the intent to effect the consecration.
So we can see that the Mass, as such, consists of more than the Eucharist. You cannot have a Mass
without the Eucharist, but a valid Eucharist alone does not guarantee that you have a valid Mass. For instance, if the priest omits the Offertory or neglects to communicate himself, the Eucharist is validly consecrated and people who received Communion have indeed received the Blessed Sacrament, but even so you have not actually had a Mass, because the Mass requires all the elements of the Sacrifice.
