We present new evidence for a special opaque interaction between phonological processes in Gua, a nearly endangered Guang (Niger-Congo) language spoken in eastern Ghana. This interaction, which was first observed by Obiri-Yeboah (2021), is between ATR vowel harmony and hiatus-resolution processes that render harmony opaque. A few properties of this interaction make it special. First, ATR harmony and hiatus resolution interact productively across arbitrary combinations of words. We show this using grammatical-yet-nonsensical Gua sentences akin to Chomsky’s (1957) “colorless green ideas sleep furiously”, which could not have been memorized by speakers. This makes the interaction a clear case of opacity acquired by speakers. Second, the interaction involves multiple kinds of opacity in different derivations – specifically, counterbleeding, (self-)counterfeeding, and the recently labeled “countershifting” (Rasin 2022) – which pose a challenge to non- serial phonological theories like Parallel Optimality Theory (OT; Prince & Smolensky 1993/2004). Stratal Optimality Theory (Bermudez-Otero 1999, Kiparsky 2000, 2015) is a serial version of OT that attempts to account for opacity by assigning opaquely interacting processes to different serially ordered strata. A central prediction of Stratal OT is that opacity should correlate with morphosyntactic structure, because strata are limited to morphological or syntactic domains (Jaker and Kiparsky 2020). Building on Obiri-Yeboah and Rose (2022), we provide new evidence that ATR harmony and hiatus resolution in Gua apply only once to the entire utterance and thus cannot be attributed to different morphosyntactic domains, suggesting that Stratal OT’s limited serialism is insufficient for solving the opacity problem for OT, and that a purely phonological mechanism for deriving opacity is needed.