Well, the quoted version being used here posits that I have "knowledge of the Predictor's infallibility" and doesn't give an error rate. So there's one counterexample, at least.

Of course, "knowledge" doesn't mean I have a confidence of exactly 1 -- Predictor may be infallible, but I'm not. If Predictor is significantly more baseline-accurate than I am, then for EV calculations the primary factor to consider is my level of confidence in the things I "know," and Predictor's exact error rate is noise by comparison.

In practice I would say that if I somehow found myself in the state where I knew Predictor was infalllible the first thing I should do is ask myself how I came to know that, and whether I endorse my current confidence in that conclusion on reflection based on those conditions.

But I don't think any of that is terribly relevant. I mean, OK, I find myself instead in the state where I know Predictor is infalllible and I remember concluding a moment earlier that I reflectively endorse my current confidence in that conclusion. To re-evaluate again seems insane. What do I do next?