I didn't compile it to try, but it looks like "%%" will get substituted as "%%" - there's no way to get a single "%" character.

And

In reply to:

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Be wary of percentages. IE turns them into characters if they are followed by numbers so that's not real great. Don't use them if possible.

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That looks like you're making URLs without escaping reserved/unsafe characters properly (IOW, IE is probably doing the Right Thing). I bet that track names containing '%' give you trouble, as well as artists like "AC/DC"...

I think the right thing to do is to make a second pass over the resultant filename, to URL-encode it.