Thanks for the replies.
I found the mistake. I’m so embarrassed. It was a missing side of sashing on a block, missed because of a mix of black fabric, poor lighting and poor eyesight. I knew I’d have a hard time but plowed onward anyway. That’s what I get, I suppose, trying to rush this one through for the holidays. Lesson learned. I’m now down for the count with a nasty bug but am ripping out seams in bed. Thanks again, everyone. And if you celebrate, have a wonderful holiday!