Not going into detail without spoiler but there is one symbol too much...
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...fourth row, fifth symbol. I know you need this one to form the number 5, but it's not part of the number 8. So you could either take another position for another number, or keep it, or anything like that.
It's just a really minor error to me, and doesn't make any problems while solving. It's just a cosmetic error in my opinion.
Imo Page 41 is not a minor error at all. For me it was the most infuriating of all puzzles so far because of that error...
I don't even know what you mean with "need for number 5" because the code is 91682... So if I might suggest an improvement not only post an error it is already bad that the 8 is 4 columns wide which is completely non standard and differs from all the other digits. However then having this erroreous extra symbol makes it super bad. I don't even get how anybode can ever guess 8 there... To me the only intuitive solution for the 4th group is 61, 67 is already a stretch and 8 is just impossible... no one writes a 8 like that. And if you take into account that there are no hints that we are looking for digits not numbers this makes it even worse. If we would assume that small errors are allowed the first 9 could as well be a 5 for example.
Another not really error but suggestions... The symbol used in 23 is kinda very hard to identify (also maybe some additional graphics to help get the "theme" might not hurt here.
black and white board, some additional figures at the bottom,...