Example:

public class Class0 {    

    public void test(int i) {           
    }
}

class Class1 extends Class0 {

    @Override
    public void test(int i) {
        int x;        
    }
}

When you try to add parameter named "x" using refactoring on method Class1.test(int i) refactoring warns you 'Name "x" is already used as name of local variable'.
But when you try to do the same (adding x parameter) on Class0.test(int i) refactoring is performed and as a result you get:

class Class1 extends Class0 {

    @Override
    public void test(int i, Object x) {
        int x;        
    }
}
with duplicated variable "x"