When I have a major issue like this, I have found that the best thing to do us to put it away and work on something else. In a few days, after I have calmed down, it is much easier to find the error. Honestly, I used to unpick right away and sometimes realized my mistake was minor or not even there afterward. Just put it away and work on your band sampler for a few days and then take it out again and go row by row to find your mistake. You can either fudge the pattern or unpick. When unpicking, you can always cut a thread and then weave in whatever is left of the thread so you don't have to unpick the entire thread.