nashorn/src/jdk.scripting.nashorn/share/classes/jdk/nashorn/internal/runtime/arrays/LengthNotWritableFilter.java

has:

    public ArrayData delete(final int index) {
        extraElements.remove(index);

but extraElements is of type SortedMap<Long, Object>

index will get autoboxed to Integer, which is unfortunately not equals to a Long, so this is a no-op.

(found by running error-prone over the jdk8 sources)