$ catt PrivAndAbstract.java public class PrivAndAbstract {
    interface P {
        private abstract void foo() {};
    }
}

$ javac PrivAndAbstract.java
PrivAndAbstract.java:3: error: illegal combination of modifiers: abstract and private
        private abstract void foo() {};
                              ^
PrivAndAbstract.java:3: error: interface abstract methods cannot have body
        private abstract void foo() {};
                                    ^
2 errors

First message is correct. Second message shows that javac at this position treats the method as abstract. Choosing one of two modifiers for error analysis is not correct and could confuse the user.