Make an assumption or guess, or accept the statement of another person who reports something. Treat this assumption or guess or report as if it were a verified fact and act accordingly. Facts are more extensional. Assumptions are abstractions at a higher level of abstracting than direct observation. Treating an assumption (higher level) as if it were fact (lower level) without being prepared to change one's assessment is confusing orders (or levels) of abstracting. With consciousness of abstracting, one can be prepared for an assumption to be wrong. But denying evidence when it is encountered is "unsane".
A: "This is a fact."
C: "Look, it's not so."
B: "That can't be." (unsane).
B: "Oops, better change the plans." (sane).
There has been some discussion of "order" versus "level" with regard to Korzybski's formulations. In general "orders of abstraction" is synonymous with "levels of abstraction" and distinguishes according to extensional versus abstract directions.
"Order of abstraction", on the other hand, seems to refer to the abstracting process, with "normal order of abstracting" referring to starting with more objective level or facts and then drawing conclusions from them - extensional orientation.
"Reverse order of abstracting" seems to be associated with "intensional orientation" - the tendency to begin with judgements, stereotypes, categorizations, etc., and subsequently disproportionally selecting evidence that supports the judgement while ignoring or denying evidence that contradicts the judgement.
For the "insane", no evidence can alter the preformed judgement, for the "unsane", only significant consequences can cause the altering of the judgement, or the temporary suspension of it. For the sane, a single bit of contrary evidence is sufficient to reverse the assumption.