United States v. Mantha, 944 F.3d 352

In 2001 Mantha sexually abused and filmed a child; in 2015–2016 he was caught searching for/viewing child pornography and was found to possess additional child-porn materials.
He pleaded guilty to three counts: production/sexual exploitation (2001), access with intent to view (2015–2016), and possession (2016).
The PSR grouped the two 2015–2016 offenses but not the 2001 production offense; parties agreed grouping was correct.
The probation officer nonetheless applied the 2016 Sentencing Guidelines (which include a 2004 amendment raising the §2G2.1 base level) to all counts under the Guidelines’ one-book/multiple-offense rules, increasing the 2001 offense AOL from 33 to 40 and raising the combined TOL and GSR (210–240 months versus pre-amendment 121–151 months).
Both parties objected to applying the 2016 manual to the 2001 offense as an Ex Post Facto violation; the district court nevertheless retained the higher offense level and imposed a 196-month sentence, saying it would have imposed the same sentence under either Guidelines calculation but giving no specific reasons for the upward variance.
The First Circuit vacated and remanded: it held that applying the later Guidelines to a pre-amendment, ungrouped offense under these facts violated the Ex Post Facto Clause, and that the district court plainly erred by failing to provide a required explanation for its upward variance.

Issue	Plaintiff's Argument (United States)	Defendant's Argument (Mantha)	Held
Whether applying a post‑amendment Guidelines manual to an earlier, ungrouped offense (via the one-book/multiple-offense rules) violates the Ex Post Facto Clause	The one‑book and multiple‑offense rules permit applying the later manual to both earlier and later offenses; commentary authorizes treating related prior conduct as relevant	Applying a later, harsher manual to 2001 conduct increases punishment retroactively and violates the Ex Post Facto Clause	Applying the later manual to a pre‑amendment, ungrouped offense that is not closely related to post‑amendment offenses violates the Ex Post Facto Clause under these circumstances (vacated/remanded)
Whether the district court’s asserted statement that it would have imposed the same sentence under the lower Guidelines renders any Ex Post Facto error harmless	The district court said it would have imposed 196 months under either calculation, so any error is harmless beyond a reasonable doubt	The court’s upward variance lacked any specific explanation as required by §3553(c)(2); without a stated reason the purportedly independent justification cannot save the sentence	Harmless‑error doctrine does not apply because the record contains no adequate, specific explanation for the substantial upward variance; remand required for resentencing

Peugh v. United States, 569 U.S. 530 (2013) (post‑Booker application of later Guidelines that increases the sentence can violate the Ex Post Facto Clause)
United States v. Booker, 543 U.S. 220 (2005) (reasoning about advisory Guidelines regime)
United States v. Pagán‑Ferrer, 736 F.3d 573 (1st Cir. 2013) (one‑book/grouped‑offenses rationale — notice fiction supporting application of later manual to earlier related offenses)
United States v. McMillian, 777 F.3d 444 (7th Cir. 2015) (applying later Guidelines to earlier ungrouped offense violates Ex Post Facto Clause)
United States v. Saferstein, 673 F.3d 237 (3d Cir. 2012) (same conclusion for ungrouped offenses)
United States v. Butler, 429 F.3d 140 (5th Cir. 2005) (contrary view relying on Guidelines commentary)
United States v. Montero‑Montero, 817 F.3d 35 (1st Cir. 2016) (vacating upwardly variant sentence where court gave no coherent explanation)
United States v. Rivera‑Gonzalez, 809 F.3d 706 (1st Cir. 2016) (vacatur where court failed to provide explanation for substantial variance)