I recently noticed that with -O1, ghc was optimizing some code if Trace.traceShowId "b" $ True then return $ Left $ Serialize.BadMagic (Serialize.magicBytes magic) file_magic else first Serialize.UnserializeError <$> Exception.evaluate (Serialize.decode rest) to always evaluate the 'else' branch. This is fixed in 8.4.2, so I'm pretty sure it's https://ghc.haskell.org/trac/ghc/ticket/13930 I assume there's no point trying to get a minimal reproduction, since it's a known issue and fixed. Still, https://downloads.haskell.org/~ghc/8.4.2/docs/html/users_guide/8.4.2-notes.h... says "incorrectly optimised, resulting in space leaks". Maybe it should instead say "incorrectly optimised, resulting in taking the wrong branch in 'if' expressions"? That's a bit more alarming, and is a stronger "upgrade to 8.4.2" signal.