I can't think of one non-verbal cue a CFI could give other than the middle finger, and that isn't going to help pass an oral question
Really?
DPE: What are the VFR cloud clearance requirements for Class E airspace below 10,000 feet?
Student: 1000 feet bel... <CFI looks up> I mean above, 500 below, 2000 horizontally.
That's probably pretty lame but it was just off the top of my head. I'm not saying that CFIs in the room were actively trying to help students out, but probably some would flinch at a wrong answer or smile when a student recited something the CFI had placed special emphasis on.
As others have said though, it's pretty easy to prevent this by having the CFI sit behind the student or at least out of his line of sight.
This is EXACTLY why I'd like to have a CFI(I) in the room during an oral.
Not to brag or anything, but that particular point was one I knew pretty well and could have defended my answer using the AIM all by my lonesome if needed. But yeah, I agree. DPEs aren't perfect either, and I'd hate to see anyone given a hard time on an oral because a DPE was wrong, when a CFI could have prevented it.
If there's a problem during a test, it'll be a "he-said, she-said" with the DPE winning no matter what ... I'd say they need to allow recording then, ESPECIALLY based on the DPE above getting corrected by a CFI.
I think an even more compelling reason was given by someone either here or on the Red Board, where the DPE was allegedly overbearing and abusive throughout both the oral and the flight part. I think most DPEs would admit error and concede the point on an issue of fact or regulations, once given the correct info, even if it was after the exam. Also, a single wrong answer is generally not enough to bust someone, and I doubt that the DPE would be wrong on enough points to make the difference between passing someone and busting them. IOW I'd be more concerned about inappropriate and unprofessional behavior, or even general mental deterioration on the part of the DPE, than a DPE busting someone because of a gap in their knowledge and then lying about it to cover their butt.