Ask your friend or a family member to stand behind you while you are sitting on the organ bench. Now you have to try to play this piece and your listener should stand there watching over your shoulder.
If you haven't ever tried this experiment, here is what would happen. If your level of fluency and proficiency with this composition is not enough for public performance, you will make at least a few mistakes, most likely a lot more. It may happen that mistakes will occur in virtually every line.
This is because your focus will be weak when someone is watching over your shoulder. Therefore, the places that still need the most work from your part will be the ones in which you make your mistakes first. It's very simple - you are as strong as your weakest link.
Yes, you can try to attempt to tighten your focus level and then fewer mistakes will occur. However, if you don't know the piece very well or if your sight-reading abilities aren't developed at the superb level, then it is very likely you will end up with at least a few mistakes.
If you knew the piece pretty well, at least well enough for public performance, then even though you would get distracted by a person standing behind you, your playing level would allow you to automate at least the most difficult places.
This way you would still be in control - your fingers and feet will do the work for you without you actively thinking about this spot when you are distracted.
This level of fluency is not easy to achieve. It requires constant repetitive practice. At each practice session, you must ask yourself, "Is this the best I can do?". Most of the time, you will see that there is still some room for improvement.
Apply this trick in your organ practice today. If you find yourself making mistakes, push through, persevere with regular, wise, slow, and repetitive practice and the time surely will come when nobody can distract you and cause you to make mistakes.
By the way, if you really want to develop unbeatable sight-reading skills, check out my systematic Organ Sight-Reading Master Course.