What are your Unknown Knowns?

Things you know but don't realize that you know (or rather, don't realize that they are in the "things other people might NOT know" category) can trip you up, especially when you're trying to explain something like a decision.

They tend to pop up as a missing context, skipped or non-explicated steps, logical leaps, and garden variety mistaken "everybody knows" assumptions (in which case it is a subtype of "things you know that ain't so" errors).

What are yours?