The source package contains a file in debian/source/ that Lintian doesn't know about. Currently the following files are recognized:
* format * include-binaries * lintian-overrides * options * patch-header
This tag is emitted in case you mistyped the name of one of the above files. If that's not the case and if the file can be legitimately be expected in source packages, please file a bug against Lintian asking for the file to be recognized.