Bear with me while I explain the background a little:

Metacity themes ("metacity-theme-1.xml") are theoretically XML files. In practice Metacity parses them using GMarkup, which isn't a fully-featured XML parser; in particular it allows some things as valid which are not valid XML.

Even among people who understand XML, it is not always well-known that two hyphens appearing in sequence in a comment will cause havoc. (This is because XML comments are in fact SGML processing instructions, which begin with "<!" and end with ">"; within these, there can be comments delimited with "--", so the full result is that "<!--" ... "-->" will allow you to add comments to an XML file.) However, GMarkup, not being a fully-featured XML parser, does not work this way, and believes that comments begin at "<!--" and go on until the next "-->". Arguably, this may be considered a bug in GMarkup, but GMarkup is not supposed to be a complete XML implementation anyway.

So, to the point: there are at least five themes on a.g.o (c2, Chiro, MetaGrip, Vista Basic) which use a double hyphen inside a comment. c2 and Chiro do so merely in order to draw a line across their markup, thus:

<!------------------------------------------------------------------>

The others do so in order to nest comments, which is clearly illegal.

The point of this bug is that it is unhelpful for these files to be invalid XML. It means it is very hard to write tests and editors, as I'm trying to do as a Metacity maintainer. The fixes for each test would be trivial and I'm entirely willing to provide patches. But perhaps, on the other hand, theme authors don't want their themes messed with, and we could say that any theme which Metacity can interpret is a valid theme.