How I know that I am intermediate - early intermediate, but still, intermediate:
My pile of pieces that are too easy is growing.
I listen to a piece and I like it, I look at the score and I think: Yes, I can play this. Then I sit at my piano, and start to play, and realise, this is too easy for me. This never happened when I still was a beginner! 
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... feeling like the pianist on the Titanic ...
Beginners who have a teacher tend to rely on outside help more. Many intermediate players would start to track their progress and be conscious their strengths & weaknesses.
Recently I showed a man a recording made 7 years ago of an old song from the 1940s. There is a big jump in the piece repeated a few times… from the melody to chords. No rest before the jump so need to land on the right notes immediately. 7 years ago I wouldn’t be playing many pieces with jumps but at least 1 piece was played successfully. The pieces you play may not get more difficult than the most difficult piece you tried but you’ll be playing more pieces at the same level.
Be aware of your mistakes and fix them like counting issues and missing sharps & flats. Other issues related to techniques depends. Executing a big jump, controlling dynamics, connecting phrases we may not be able to do yet.
Part of fixing mistakes is developing a good ear. This means not to take sheet music at face value even if we’re good readers. Over the years I downloaded pieces and would find spots that doesn’t sound right. 1 piece missing a flat on the D, another missing a flat on the B. Good readers would play the mistakes as is. Someone like me would find a recording online and hear the spots that don’t sound right. My weakness is reading. Being a good listener compensates my reading.
thepianoplayer416 Part of fixing mistakes is developing a good ear. This means not to take sheet music at face value even if we’re good readers. Over the years I downloaded pieces and would find spots that doesn’t sound right. 1 piece missing a flat on the D, another missing a flat on the B. Good readers would play the mistakes as is. Someone like me would find a recording online and hear the spots that don’t sound right. My weakness is reading. Being a good listener compensates my reading.
Curious. How do you know the score is incorrect? I have seen MuseScore scores someone has done of pieces written by well-known composers and they have a few incorrect notes, but I know that because I can compare the piece with published scores on IMSLP, for example. Sometimes our ears tell us to correct a dissonance we hear, but that's not what the composer wants us to do.
Stub As a MuseScore user myself, I find that scores notated by others have errors. After hearing at least 1 version of the score, you pick out wrong notes easily.
The last piece I downloaded from MuseScore was a piano arrangement of a singing piece. The original probably has dynamic markings on top of the staff and the lyrics under the staff which is acceptable. A piano score without the lyrics having dynamic markings on top looks odd.
Don't assume a download from IMSLP is free of mistakes. I downloaded a copy of Bach Goldberg Variations. In mvt 1 (Aria) there was 1 bar with half a beat too many. Counted a few times and then entered the bar into MuseScore to check if it was my counting mustake. Still came out as half a beat too many. An IMSLP score can came from a bad photoopy. The last one downloaded has notes that are unclear with fade circles. Since the piece has repetitions you can tell by comparing sections that sound the same or similar but with some notes unclear.
The discrepancies over sharps & flats: A piece you know well you can hear if a note sounds wrong. A pieces that was originally for singing arranged for piano there are different versions. You find note differences between versions as long as an arrangement sounds ok.
The last piece with a missing flat on the B came from an orchestral score with a piano part. Checked the note with violin I from the strings instruments above (violin I, II, viola & cello). Bar 149 Vln 1 says Bb, the piano part says B so have to assume 1 of them is not right. The piano part you can kind of tell the B is missing a flat. Bar 148 before has a flat in front of the B. Bar 150 has a natural in front of the B. Bar 149 between the 2 has nothing (nothing in the key signature) means the note is a B. The bar after has a natural as reminder so you assume the B in bar 149 with nothing next to it has a missing flat.
Take some of the most played popular piano arrangements, let's say The Beatles. Predominantly an electric guitar band. Do people really believe that Lennon and Machartney sat down and accurately transcribed what they finally recorded into 100 percent piano sheet music. Someone not even in the band likely did this.
And is this the recorded version, or what they would play in a live performance. I'm assuming most people are aware that few bands play a live version identical to what they produce in a recording studio. And if people want to reproduce someone's live performance Which nights performance. Many musicians vary what they play every night. Elton John is well known for this. And why wouldn't have musicians done this also 200 years ago.
Sydney Australia
Retired part-time piano technician
thepianoplayer416 Also MuseScore user/editor here. Very often I find many arrangements of the same piece, and often none of those is "perfect". I can hear in every one a mistake, but a different mistake.
Then I get the best version and compare the mistake-section to the other versions. There is a good chance that at least one of the others noted that section correct. And then I correct my downloaded version.
Finally I adapt the staff-sizes and make line- and page-breaks to fill up the pages evenly.
I hate it, if a score has 5 lines on page one and 3 lines on page two. Or if measures are crammed, but then there is empty space somewhere else, or the last measure is only one note for an entire staff line.
To get a bit back on topic:
I found it much easier to discover mistakes in the score as a beginner than as an intermediate, because most beginners' pieces have much simpler harmonies.
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... feeling like the pianist on the Titanic ...
WieWaldi
You’re less likely to find mistakes in a score from a commercial printing company. The last piece I worked on did find a missing Bb. People who upload MuseScore pieces don’t notice or fix mistakes.
I also reformat my pages so that I don’t get 1 or 2 bars hanging at the end of a section. A section would always end at the end of a line.
thepianoplayer416 You’re less likely to find mistakes in a score from a commercial printing company.
Why is a commercial printing companies arrangement going to be better than some independent persons arrangement? Both, even your own, are just interpretations of perhaps just one take of how someone else recorded something, maybe not even having a piano in the mix, on any given day.
Sydney Australia
Retired part-time piano technician
JohnCW Why is a commercial printing companies arrangement going to be better than some independent persons arrangement? Both, even your own, are just interpretations of perhaps just one take of how someone else recorded something, maybe not even having a piano in the mix, on any given day.
The reference is to original scores, not arrangements. For arrangements, like you say, anything goes.
Stub The reference is to original scores, not arrangements. For arrangements, like you say, anything goes.
The overwhelming majority of published popular music is an arrangement. At no time did the band or orchestra actually play what is printed on the page.
Sydney Australia
Retired part-time piano technician
JohnCW A lot of great info here - @JohnCW is totally correct in saying that the original artist almost never produces the sheet music for their song. Even for more academic music (e.g. classical) the composer's score isn't going to be what is sold commerically. There are always substantial edits required.
Often, as John says, that is transferring the song from guitar->piano or similar. Sometimes it's about reducing needless complexity (does that exact improvised lick need to be captured, or is it more important to get the main "feel" of the song across?). Sometimes it's about putting it in a more standard key.
It is definitely possible to make exact/precise transcriptions, if that's what you need (as many of our clients do). But commercially release sheet music is extremely rarely that precise.
