After weaving quite a few inches into the towel I thought something looked off and discovered I was missing a thread!
If you find the pin in the picture you can see that part of the pattern just looks a bit off. Follow that line up to where the thread finally is threaded and you can see where the pattern got corrected after I added a new warp thread.
In case anyone new to weaving is wondering how you recover from a mistake like this, measure out a new warp thread, thread it through the reed and heddle, and then hang it off of the back of the loom with a weight. I wrap the end of the warp around the weight until it hangs properly, and unwind it a bit every few times I advance the warp.
Then wrap the other end around a pin and pin it to the fabric. After enough rows eventually tension will keep it in place.
In northern Sweden you can almost always find one error in old weaves, but no mistakes. To make a perfect weave was considered prideful.