When somebody sends me something that's advertised to be XML, the first thing I do is run xmlwf (James Clark's expat parser) on it and then open it up in Internet Explorer. (They never disagree, but I do both anyhow). If they tell me the XML is broken, I call up the data source and say “Expat and IE both say your XML is broken.”, and every time, they say “Oops” and fix the problem.
