Rhino has support for E4X extensions, such as attribute get operator a.@b. It is parsed correctly as XmlMemberGet node, but toSource() call causes IllegalArgumentException, thrown from AstNode.operatorToString(), which is given '.' or '..' token.

Following snippet resembles this issue (default CompilerEnvirons is used):

String script = "a.@b;";
Parser parser = new Parser(environment);
AstRoot root = parser.parse(script, null, 1);
/* up to 1.7.9 following will throw IllegalArgumentException */
root.toSource();

I have prepared a pull request, it will be opened right after this issue is created.