I made chicken & andouille gumbo last night. I made a roux to a deep mahogany, added my trinity, cooked down a little, added seasonings then hot chicken broth, brought to a boil (rolling, didn't mean to, but walked away while on the phone), turned down and simmered. Added chicken (that'd been cooked separately for the aforementioned broth) and browned andouille. Oh, then frozen okra a bit before serving, but the broth was already weird before that addition.
So here're the further clues: I skimmed off about 1/2cup of oil (3/4c to 1 cup flour that'd been used in the roux). The broth had an almost metallic bitter taste.
My step-dad had the same problem not long ago and we suspect that the roux "broke" during that "oh shit, it's about to boil over!" rolling boil, as his did the same.
Are we on the right track, or no? Can a roux even "break" the way any other flour-based sauce can? It seems likely, but almost unlikely at the same time.