The Importance of Consistency in Storytelling

Marius: I'm able to hang lanterns on by just saying, you know, when the character last quoted this particular maxim, you got it wrong. The other one is, I will sometimes use these inconsistencies and mistakes as what they call it, painting a happy accident. For me, the historical setting that I've chosen is not window dressing - it's absolutely integral. How do you decide when something should be a hidden version of our world or how do you decide if it needs its own universe?