the following erroneous class compiles with no errors with fastjavac:
public class FastJavacBugTest {
    public void func(){
        System.out.println("func");
    }
    
    public static void main(String[] args) throws Exception{
        new Inner().func2();
    }
    
    public static class Inner{
        public void func2(){
            func(); 
        }
    }
}

when running the class the following error appears:
java.lang.VerifyError: (class: FastJavacBugTest$Inner, method: func2 signature: 
()V) Incompatible object argument for function call
        at FastJavacBugTest.main(FastJavacBugTest.java:7)
Exception in thread "main" 

when compiling with javac.exe (modern) the following compilation error appears:
FastJavacBugTest.java [12:1] non-static method func() cannot be referenced from 
a static context
            func(); 
            ^
1 error
Errors compiling FastJavacBugTest.