FRE, Sentencing.
Although trial court's interventions in questioning of witnesses were imprudent, poorly conveyed, and went beyond the pale, they do not constitute plain error, as they were not ultimately dispositive of the verdict.
Where a judge says that since good reasons must be given for variances, the sentencing guidelines are effectively compulsory -- the sentence is therefore procedurally unreasonable; further inquiry into substantive unreasonableness is foreclosed.
http://www.ca4.uscourts.gov/Opinions/Published/134828.P.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.