If an unresolved variable is encountered, the plugin simply does not replace the variable in the target file. If this unresolved variable however ends in ".file}" it will evaluate to a file object that targets the current pom. This results in the replacement being the complete path to that pom (in the 2.1 version of the plugin this results in a ClassCastException). The workaround is, of course, not to filter the affected files. Though this will not work if other variables in the affected files do need to be replaced.