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.