In United States v. Johnston, 220 F.3d 857 (8th Cir. 2000), the defendant was convicted and sentenced to a life sentence pursuant to 21 U.S.C. § 841(b)(1)(a), which states that a person convicted under § 841(a) of manufacturing, distributing, dispensing, or possessing a controlled substance or counterfeit substance and who has "two or more prior convictions for a felony drug offense that have become final, ... shall be sentenced to a mandatory term of life imprisonment without release."
In holding that a mandatory penalty could result from convictions incurred prior to the date of the statute's enactment, the court reasoned that a court accepting a guilty plea could not warn defendants of a mandatory penalty prior to the legislative enactment and that the statute itself constituted fair warning of the mandatory penalty. United States v. Johnston, 220 F.3d at 862.