Burns v. State
In Burns v. State, 703 S.W.2d 649, 652 (Tex.Crim.App. 1985), the Court found error harmful under the "some harm" standard where the witness was an accomplice as a matter of law, the accomplice witness testimony was corroborated only by the defendant's confession and fruits of that confession, and the defendant was challenging the voluntariness of his confession.
The Court reasoned that, if the jury found the confession to be involuntary, then the confession and its fruits would be disregarded, and the accomplice testimony would then be uncorroborated.
Burns relied upon the pre- Almanza v. State (1984 approach disavowed in Saunders, but much of its reasoning is still valid. After Saunders v. State (1991), though, the voluntariness challenge to the confession would be relevant, but not by itself determinative; if the voluntariness challenge were especially weak, or some other factor mitigating against harm were present, the confession might still be considered sufficient corroborating evidence to render the accomplice witness charge error harmless.
In Burns the voluntariness challenge provided a rational and articulable basis for disregarding the fruits of the confession.
Of course, even with a rational and articulable basis for disregarding the corroborating evidence, other factors may affect the harm analysis - as in Medina, where the weakness of the evidence of accomplice status, combined with the significant non-accomplice evidence, rendered the error harmless.