Talking about Opera

I am with a group here and have a lot of opportunity to talk about opera. Those of us who have been watching opera a long time have had the opportunity to see the operas presented as they are originally intended.

But if you are new to the opera, how would you know that Snow White and the Seven Drarfs do not normally appear in Die Meistersinger? How would you know that it isn't supposed to be a dream? How would you know that Don Alfonso doesn't normally die of poison at the end of Cosi?

I am hearing a lot of confusion. Why did that happen? The only possible answer is Who knows?