I've come across some particularly terrible ones:
Ganondorf battle (windwaker): the sheet looks wrong and has [title] at the top of each page. The notes are accurate, but there should be harmonies instead of single notes.
Hidden Village (Twilight Princess) Has no harmonies at all, when the song clearly has them (same arranger above) and a few note errors and the midi has a gap at the repeat
And the worst:
Rise Above (FE: Sacred stones) Besides having single notes where chords might be more appropriate, measure 20 is the single worst measure of music on the entire site. It sounds like the arranger was just writing in random notes and submitted it like that.
And of course, The Halberd (Kirby super star) has accurate rhythms, but only about 85% of the notes are accurate.
I might fix up a few sheets in my spare time, but I still want to finish the super metroid soundtrack and get some fire emblem awakening in there.