Failing test case below. Using the constructor AbsoluteDate(DateComponents, TimeComponents, TimeScale) adds one second when the time is during a leap second. Seems to be due to UTCScale.offsetToTAI expects that timeComponents is 23:59:60.5 during a leap second, but it is actually 24:00:00.5. So the bug might actually be in the TimeComponents constructor.

    @Test
    public void testLeap() {
        TimeScale utc = context.getTimeScales().getUTC();
        // During 2012 leap second
        final DateComponents dc = new DateComponents(DateComponents.MODIFIED_JULIAN_EPOCH, 56108);
        final TimeComponents tc = new TimeComponents(86400.5);
        final AbsoluteDate date = new AbsoluteDate(dc, tc, utc);
        final AbsoluteDate date1 = new AbsoluteDate(dc, utc).shiftedBy(86400.5);
        Assert.assertEquals(date, date1);
    }

Same as #247 (closed) but for the constructor. Strangely it seems that #247 (closed) was fixed by working around this bug in the constructor.