I spent hours threading a loom for twill, only to learn I had three errors in the cloth, places where two threads were in the same shed side by side.
After a couple more hours, I could say that I'd followed the draft in the book exactly. Once I understood what had happened, I predicted that I'd find a fourth error. It turned up exactly where I thought it would be, where one block followed another. I formed a theory about what it would take to change the blocks to avoid the errors, by flipping the third block on the vertical axis.
My weaving teacher had previously taught me about the rules of twill but I didn't apply them to this draft. I assumed that what you see is what you do. The same day after I discovered the errors, I read about Summer and Winter patterns in Mary Meigs Atwater's American Hand-weaving. It changed my understanding about what it would take to change the book's draft and avoid the error. I now think it would have been a matter of overlapping the blocks in question. I think the book with the pattern, Davison's A Handweaver's Pattern Book, assumes a certain level of experience and understanding.
While it's good I have been able to think through the problem and come up with a solution to present to my weaving teacher for review and critique, it would have been better to have perceived the need for a change sooner.