Today the Xerces XML parser gave me this bogus fatal error:

The attribute 'aMixedCaseAttr' is already used in element 'anElement'

I was sure I'd seen it before, and sure enough it's bugzilla bug #26627. Turns out it's a bug in Xerces v2.4, It sometimes gives this error when the XML document contains mixed-case attributes. The only solution is to upgrade to a newer version.