"Might not" or mightn't

We might be so much oftener together - mightn't we?

Hard Times by Dickens, Charles 

Whether that might be, or mightn't be, is a thing as can't be looked into now, without putting your sister on the Rampage; and that's a thing not to be thought of, as being done intentional.

Great Expectations by Dickens, Charles 

"Well," I said, half laughing, "there is the danger that, after all, we mightn't agree."Might not" suggests you do not know if something happens.
Examples:

* Jack might not have the key. Maybe he does not have the key.