Ah! My nemesis!
A while ago I was in Vegas, went through MGM Grand quickly, backcounting shoes a couple of times.
Then, a week later, I was talking to someone about the trip, and mentioned playing their 8D games. To which he responded "MGM Grand only has 6D games". I even called up the Grand and asked, and yep, 6D only. It was a pretty horrible mistake, but at least it only caused me to underbet when I wonged into the shoes.
I think I was putting too much stock on the discard trays, and not enough on the cards themselves.