This is the first error I spotted: I had used a heddle on the wrong shaft. The texsolv heddles do not undo - I looked really closely because if it had been possible to untie it I could have simple moved the heddle. I contemplated for a while the idea of cutting the warp off and then re-threading. I felt really depressed by that idea. It would take me ages.
Then I remembered reading somewhere, probably first in Osma Gallinger Tod's Manual of Helpful Hints for Handweavers, the solution of making a new heddle and tying it on the correct shaft. I got a couple of lengths of warp cotton and tied two overhand knots for the heddle eye, then tied it on the top shaft. Tying on the bottom shaft I didn't manage so easily. I did this from sitting under the loom and reaching across. Big mistake. I pulled a muscle in my back that stopped me weaving for a couple of days. It didn't help that I managed to tie the heddle to the wrong shaft first go, and used a knot that wasn't easy to undo. I consulted my friends in the Online Guild of Weavers Spinners and Dyers about this - and received the useful suggestion that on some looms you can use a paperclip around the bottom shaft. Well, my shafts are too wide for that - but the concept in this suggestion, of a loop around the bottom shaft to tie on to is a great idea. A slack loop of knotted thread would work fine - next time. Anyhow, I was rather proud of my achievement in getting this new heddle made and getting rid of the mistake without having to cut the whole warp off, so I took these commemorative pictures.
And, next picture, the single cut thread is knotted to a length of (red) thread for attaching into the woven cloth. I had no problems with this, I didn't know that new heddle was there once I got weaving and the tension of the thread was as good as the rest of the warp.