Grah, mysterious coverage error. I have a theorem that coverage-checks when I have a "hole" term at the type the theorem is supposed to output. The theorem consists of two mutually recursive statements A and B. If I add clauses to A (hoping to eventually remove the "hole" cases), then there are new missing cases reported for B. Insane!
ETA: Have found a way around it this time... the solution unfortunately isn't that general.