I have a distant memory that in geometric proofs, there was a term that the teacher could insert at the end if the result has an error.
Essentially saying, “result is absurd so there must be a mistake; re-do”
Opposite of “Q.E.D.”
Does this ring a bell with anyone?
According to this, a “??”
False? Contradictory? Absurd? Bogus? Wrong?
Note that QED does not mean “true” or “correct”. Was it a Latin phrase you were looking for?
“W.T.F.”? 
Probably not what you’re looking for but traditionally in Euclidian geometry, Q.E.D. (quod erat demonstrandum = “what was to be demonstrated”) is used at the end of the proof of a theorem. Q.E.F. (quod erat faciendum = “what was to be done”) is used at the end of a problem or exercise that doesn’t involve proving a theorem.
Well, I taught math for nearly 40 years and, while such a term would have been all too useful, I never knew it.
One of my profs used “Badly Substantiated” or “BS” for short.
Non Sequitur - “it does not follow”
Or perhaps -
Proof by contradiction. In logic, proof by contradiction is a form of proof that establishes the truth or validity of a proposition by first assuming that the opposite proposition is true, and then shows that such an assumption leads to a contradiction. reductio ad impossibilem.
“I think you should be more explicit here in step two.”
That’s it!
Isaac Barrow, one of Isaac Newton’s teachers, would end such a proof with “Quod est absurdum” - “Q.E.A.”
See the section on “notation” in the wiki article:
AFQ.
From a false proposition, anything follows.
Or you can make up your own 
QFE: quod falsum est. “Which is false”
If you are writing intelligently, you would state that the goal is to get a contradiction if the claim is false:
Step 1. Assuming X is false, we will show that it leads to a contradiction.
Steps 2 … n-1 (various stuff)
Step n. And therefore we see that 0=1 which is the contradiction we sought. Q.E.D.
Some text as to why the contradiction is aimed for is helpful so there would be no need to end on a negative conclusion. You phrase it as a positive goal.
quod errata demonstrandum
When my father was teaching English, he had a rubber stamp made up that he would use to indicate that a statement made is blatantly false and needs to be rewritten.
“Piffle.”