ADDITIONAL SYSTEM INFORMATION :
Windows 10, Java "1.8.0_191" / "1.8.0_202" / "11.0.2".

A DESCRIPTION OF THE PROBLEM :
Math.acos(1.00000000000000001) return 0.0 (almost correct).
But Math.acos(1.000000001) return NaN.
Why ?
//not found similar bugs existing.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
public static void main(String[] args) {
        System.out.println(Math.acos(1.000000001));//return NaN ?
        System.out.println(Math.acos(1.00000000000000001));//return 0.0 (almost correct)
}

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
0.0
0.0
ACTUAL -
NaN
0.0

---------- BEGIN SOURCE ----------
public static void main(String[] args) {
        System.out.println(Math.acos(1.000000001));//return NaN ?
        System.out.println(Math.acos(1.00000000000000001));//return 0.0 (almost correct)
}
---------- END SOURCE ----------

FREQUENCY : always