I was working on an XML tester module for a colleague of mine when I discovered a small XML server bug in Drupal (namely this issue: http://drupal.org/node/231132). The problem in question was that in core we use t() function to print the error messages into the returned XML, thus enabling localized errors. This would be cool, but until now the error messages had a placeholder in them, to print the variables. Using the '%' placeholder, all the variables got a tag around them, and so the parsing failed when reading the returned XML.

Thanks to a typ from Jorrit on the issue I learned, that the placeholder '@' solves this. So if you don't want to have your variable emphasized in the t function, just use the @ placeholder and all should be well.