I have woven about 18 inches when I noticed the threading error. It is actually captured very nicely in the header photo! Had I really looked, I would have seen it. The error was just about in the center of the warp. What should have been a threading of 1-2-1-2 was 1-2-2-1. So it was a simple fix: cutting the two errant ends and exchanging the heddles so the threading is correct.
Then just to top it off, my right selvedge broke so I had to tie that off too.
Towel One will have the error for a bit. Oh I did not even consider unweaving! Nor starting over and cutting that off. Towel One will stay with me most likely.