How much unfairness equals "reversible error"?

In Guerrero v. Smith,  we got a demonstration of just how unfair a trial can be and not require reversal:  at least in Michigan and in this politicized environment.  Guerrero's injury claim was rejected by the jury and he appealed, alleged multiple errors at trial. 

The Court of Appeals reviewed his claim and agreed with him that Defense Counsel's alleged repeated remarks concerning his prior marijuana use would not be allowed if the purpose was to prejudice the jury, but concluded that this was not Defense Counsel's purpose in making these remarks.  The Court also agreed that Defense Counsel erred in questioning another witness about Guerrero's past marijuana use.  It termed this error "harmless".   It also termed "harmless" Defense Counsel's outrageous statement that Plaintiff's attorney had assisted plaintiff in "faking it [and] these sorts of things" based on Defendant's admission of a letter from Plaintiff's attorney to his doctor which did nothing more than "set out to find and obtain favorable evidence".  [So far as we know, that is explicitly what every lawyer is hired to do and a function that the court depends on the attorneys to accomplish competently in an adversary system.}

As if that weren't enough evidence of a tainted trial, the Court also admitted that Defense Counsel erred by improperly raising the issue of wage loss benefits and lack of health insurance to suggest an improper motive or malingering by Guerrero.  {"It is true that attorneys must avoid injecting the issue of insurance coverage and benefits into trial."]  The Court did not consider that this error necessitated reversal, either.  In addition, it did not fault the trial court for refusing to give three jury instructions sought by the Plaintiff which were "lifted word-for-word from our Supreme Court's decision in Kreiner"  [in one case] and which all would have "properly informed the jury of the applicable law."

Despite these several errors, the Court of Appeals described this as a "well-tried case" and refused to set the verdict aside.  It makes one wonder just how much error it would take, and how many underhanded tactics would be enough to render a Defense verdict "unfair".