I ran the potato math and even assuming 100% conversion efficiency (which isn%u2019t likely) there%u2019s no way you got 1.066 from the mash.
I agree. Impossible. You'd have to have 105% mash efficiency or something. I've seen 94% before, but never 105%.
(EDIT: If you saw my previous response above, nevermind it, deleted.)
You're especially going to expect a huge reduction in brewhouse efficiency from this point, if 1.5 gallons of wort was left behind in the mash tun. Wasted sugars.
One of the two readings is completely wrong, if not both. So, check all your calibrations. What does your refractometer read in plain water? What does your mash thermometer read in boiling water? Ice water?