In GuideOne Elite Ins. Co. v. Fielder Road Baptist Church, 197 S.W.3d 305, 307 (Tex. 2006), the Court noted that some appellate courts have recognized such an exception, but those courts "have drawn a very narrow exception, permitting the use of extrinsic evidence only when relevant to an independent and discrete coverage issue, not touching on the merits of the underlying third-party claim." Id. at 308-09.
Although not adopting this narrow exception, the GuideOne Court expressly rejected "the use of overlapping evidence as an exception to the eight-corners rule because it poses a significant risk of undermining the insured's ability to defend itself in the underlying litigation." Id.