The majority of tags from legit music I've bought have been incorrect.
The most common problems are:
1. Confusing composer with artist. If the song is a remix, the artist is the remixer. The original artist is the composer.
2. Genres are fuzzy. Lots of songs fit into many genres. Picking a single genre is inaccurate at best. Sadly the id3 spec only lets us pick one, so I comma separate them out of protest. Wikipedia does this too. Look up an album, see many genres, not one.
3. The infamous "Various Artists" artist. Likewise with genres, I comma separate artists because the id3 spec doesn't let us add multiple artists. Although this is actually becoming more common with legit purchased music too.
4. Bad metadata. Even legit purchased music sometimes has errors, typos, bad punctuation, etc. It always enrages me when legally purchased music has these kinds of metadata errors.
5. Quality of downloaded music. Lossless or GTFO. This is very rare. I usually have to buy actual CDs and rip them to get that kind of quality when doing it legally.