How to get more accurate performances with difficult passages (eg. the end of Chopin Nocturne in C# minor)

Many of the activities we do are either purely physical (do enough "reps" and your muscles get stronger) or purely cognitive (encounter and understand a new concept). Practicing an instrument has to be a combination of the two. It's true that you won't get anywhere without actually doing the physical motions, and that it takes some repetition. But repeating the motions blindly, with no cognitive engagement, is at best inefficient and sometimes counterproductive. I call this the "Super Mario method"—run along, fall off the cliff, try again and hope for the best.

When you "mess up," chances are there's a reason. Figure out why and you have a better chance of doing something about it. Is it an awkward finger crossing? an extension? Is it conceptual, based on the way you're grouping the notes in your mind? Distracted by motion in the other hand? Something about the angle of wrist or elbow? Analyze your motions and thoughts and see if you can diagnose the mistakes. (Sometimes you can't, and that's ok.) Then once you have an explanation, you can make accommodations: Extend a bit more intentionally for an extension, prepare a finger crossing, etc.

Side note: We spend so much time staring at the sheet music that it's easy to think that those dots "are" the music, and to let even their visual shapes on the page shape our thinking. But try to think about the physical and auditory realities of making the music. Two dots might be next to each other on the page, and be executed by two neighboring fingers; two others might be just as close but involve a much bigger motion like a crossing or even change of hands. Look away from the dots once in a while to interrogate the actual mechanisms of music-making.

I have around 50 percent chance to mess up the long fast scale at the end ... Is there anything special I need to do in my practice beyond just repeating this scales many times to get more consistent?

Remember that every time you play the scale incorrectly you are reinforcing the incorrect execution. Determine how to minimize this.

One approach is to practice more slowly to improve the percentage of correct iterations. Find the tempo at which you can play it correctly 90% of the time. Once your average is above 90%, increase the tempo slightly. You may need to keep at this for a couple of weeks or even months.

A common mistake is to practice difficult passages repeatedly until you get it right and then move on to something else. This approach is doomed to failure. Consider: if you play the passage three times with errors and once correctly then you have just reinforced the incorrect performance three times as much as you have reinforced the correct performance. I had a teacher in secondary school who, after we had fixed something in rehearsal, always had us repeat it three or more times correctly before moving on. It was a bit tedious, so he made a bit of a joke of it, but it worked, and it worked very well.

First, you can’t become 100% completely consistent, we are all human and will make mistakes from time to time. Second, there are things you can do to improve, and simple repetition won’t do as much as other things.

• Josh Wright and others on YouTube have practice techniques that help with this kind of thing. For Wright, I’d look for his “play, press, release” technique.
• Musicians on all instruments will improve their fast passages by practicing them with accents in various places, like every third or fourth note. This gives you landmarks to hit along the way. A similar technique is to practice the passage with different note values, such as doubling every other note value (thank you, fdomn-m).
• Work to have a flow to the run, similar to the accent pattern but not as audible for the listener. In other words, divide the run into smaller groups of notes in your mind and think about it as playing each group leading to the next.
• Do not “practice” at full speed. Use a metronome and only let yourself work on it at a painfully slow tempo at which you can be consistent. If you mess up, slow down the metronome and do it again. If you have to set the metronome to 40 BPM and play one note per click to make sure it’s consistent, that’s what you do. Do that every day and slowly increase the metronome over time. Before you can play it right quickly, you have to play it right. Don’t try to correct errors at speed, speed up your error-free playing.
• When you play it correctly, immediately play it correctly at the same tempo in the same way at least two more times to fix the correctness in your memory. Some recommend at least ten correct repetitions before moving on. I've found three to five works for me.
• Basically the same as Andy’s answer: make sure you don’t have any incorrect technique in your playing. Look for tension and extraneous motion.

Overall this is an area where even hobbyists benefit greatly by having a competent teacher.

Playing the exact passage over and over might seem like the path of "practice makes perfect" but I see two problems in that approach:

• If you make mistakes in the process, you just reinforce whatever is going on leading to mistakes.
• You probably will become "numb" to the music and the movements. You will become bored. You will loose focus and musical "intent."

When you get to a certain level on something. When you hit a real plateau in your current ability. It is often better to move on to something else and later return to previous work. That's true for many things not just piano.

I suppose what that really means is, it isn't the specific Chopin piece that is the issue, but overall scale technique. You might woodshed technical scale exercises. Maybe get scale based etudes. Or just move on to the next piece. Maybe select new pieces that feature more scale work.