That's interesting! I have tried to get false positives from pangram and failed, so I trusted it a bit more than any of the others, although I generally just rely on my own intuition. I am curious what your false positive samples looked like, if you're willing to share.

(I'm less interested in false negatives; I have successfully produced those myself.)

I'll try to pull them up for you, I'd have to go back and find them on my computer.