I've been sorting through a ton of duplicates lately, and there are a significant percentage that haven't been matched correctly due to discrepancies like "Doobie Brothers" vs "The Doobie Brothers", or different versions of apostrophes in words like "There's". It seems like a lot of what was missed would be cleared up by applying the "ignore words" and "ignore diacritics" settings when comparing tags, if the user has those configured.
One other thing that breaks matches is parentheses, like "Don't You Forget About Me" vs "Don't You (Forget About Me)". Some handling for that, too, might be helpful. Everything else I've noticed seems to involve actually incorrect tags or personal tagging preferences that would be harder to account for.