The failing test cases:

src/test/java/org/joda/time/TestDateTimeZone.java:243-288
public void testForID_String_old() {
    Map map = new LinkedHashMap();
    map.put("GMT", "UTC");
    map.put("WET", "WET");
    map.put("CET", "CET");
    map.put("MET", "CET");
    map.put("ECT", "CET");
    map.put("EET", "EET");
    map.put("MIT", "Pacific/Apia");
    map.put("HST", "Pacific/Honolulu");
    map.put("AST", "America/Anchorage");
    map.put("PST", "America/Los_Angeles");
    map.put("MST", "America/Denver");
    map.put("PNT", "America/Phoenix");
    map.put("CST", "America/Chicago");
    map.put("EST", "America/New_York");
    map.put("IET", "America/Indiana/Indianapolis");
    map.put("PRT", "America/Puerto_Rico");
    map.put("CNT", "America/St_Johns");
    map.put("AGT", "America/Argentina/Buenos_Aires");
    map.put("BET", "America/Sao_Paulo");
    map.put("ART", "Africa/Cairo");
    map.put("CAT", "Africa/Harare");
    map.put("EAT", "Africa/Addis_Ababa");
    map.put("NET", "Asia/Yerevan");
    map.put("PLT", "Asia/Karachi");
    map.put("IST", "Asia/Kolkata");
    map.put("BST", "Asia/Dhaka");
    map.put("VST", "Asia/Ho_Chi_Minh");
    map.put("CTT", "Asia/Shanghai");
    map.put("JST", "Asia/Tokyo");
    map.put("ACT", "Australia/Darwin");
    map.put("AET", "Australia/Sydney");
    map.put("SST", "Pacific/Guadalcanal");
    map.put("NST", "Pacific/Auckland");
    for (String key : map.keySet()) {
        String value = map.get(key);
        TimeZone juZone = TimeZone.getTimeZone(key);
        DateTimeZone zone = DateTimeZone.forTimeZone(juZone);
        assertEquals(value, zone.getID());   // <-- TC fails here
//            System.out.println(juZone);
//            System.out.println(juZone.getDisplayName());
//            System.out.println(zone);
//            System.out.println("------");
    }
}

The faulty code:

Line is covered but NOT in slice
Line is covered and in slice
Line is in slice but NOT covered
src/main/java/org/joda/time/DateTimeZone.java:558-598
private static synchronized String getConvertedId(String id) {
    Map map = cZoneIdConversion; 
    if (map == null) { 
        // Backwards compatibility with TimeZone.
        map = new HashMap(); 
        map.put("GMT", "UTC"); 
        map.put("MIT", "Pacific/Apia"); 
        map.put("HST", "Pacific/Honolulu");  // JDK 1.1 compatible 
        map.put("AST", "America/Anchorage"); 
        map.put("PST", "America/Los_Angeles"); 
        map.put("MST", "America/Denver");  // JDK 1.1 compatible 
        map.put("PNT", "America/Phoenix"); 
        map.put("CST", "America/Chicago"); 
        map.put("EST", "America/New_York");  // JDK 1.1 compatible 
        map.put("IET", "America/Indianapolis");            //<-- BUG!! 
        map.put("PRT", "America/Puerto_Rico"); 
        map.put("CNT", "America/St_Johns"); 
        map.put("AGT", "America/Buenos_Aires");            //<-- BUG!! 
        map.put("BET", "America/Sao_Paulo"); 
        map.put("WET", "Europe/London");                   //<-- BUG!! 
        map.put("ECT", "Europe/Paris");                    //<-- BUG!! 
        map.put("ART", "Africa/Cairo"); 
        map.put("CAT", "Africa/Harare"); 
        map.put("EET", "Europe/Bucharest");                //<-- BUG!! 
        map.put("EAT", "Africa/Addis_Ababa"); 
        map.put("MET", "Asia/Tehran");                     //<-- BUG!! 
        map.put("NET", "Asia/Yerevan"); 
        map.put("PLT", "Asia/Karachi"); 
        map.put("IST", "Asia/Calcutta");                   //<-- BUG!! 
        map.put("BST", "Asia/Dhaka"); 
        map.put("VST", "Asia/Saigon");                     //<-- BUG!! 
        map.put("CTT", "Asia/Shanghai"); 
        map.put("JST", "Asia/Tokyo"); 
        map.put("ACT", "Australia/Darwin"); 
        map.put("AET", "Australia/Sydney"); 
        map.put("SST", "Pacific/Guadalcanal"); 
        map.put("NST", "Pacific/Auckland"); 
        cZoneIdConversion = map; 
    }
    return map.get(id); 
}

Other code:

src/main/java/org/joda/time/DateTimeZone.java:301-338
public static DateTimeZone forTimeZone(TimeZone zone) {
    if (zone == null) {
        return getDefault();
    }
    final String id = zone.getID();
    if (id.equals("UTC")) {
        return DateTimeZone.UTC;
    }

    // Convert from old alias before consulting provider since they may differ.
    DateTimeZone dtz = null;
    String convId = getConvertedId(id);
    if (convId != null) {
        dtz = cProvider.getZone(convId);
    }
    if (dtz == null) {
        dtz = cProvider.getZone(id);
    }
    if (dtz != null) {
        return dtz;
    }

    // Support GMT+/-hh:mm formats
    if (convId == null) {
        convId = zone.getDisplayName();
        if (convId.startsWith("GMT+") || convId.startsWith("GMT-")) {
            convId = convId.substring(3);
            int offset = parseOffset(convId);
            if (offset == 0L) {
                return DateTimeZone.UTC;
            } else {
                convId = printOffset(offset);
                return fixedOffsetZone(convId, offset);
            }
        }
    }
    throw new IllegalArgumentException("The datetime zone id '" + id + "' is not recognised");
}
src/main/java/org/joda/time/DateTimeZone.java:132-162
public static DateTimeZone getDefault() {
    DateTimeZone zone = cDefault;
    if (zone == null) {
        synchronized(DateTimeZone.class) {
            zone = cDefault;
            if (zone == null) {
                DateTimeZone temp = null;
                try {
                    try {
                        String id = System.getProperty("user.timezone");
                        if (id != null) {  // null check avoids stack overflow
                            temp = forID(id);
                        }
                    } catch (RuntimeException ex) {
                        // ignored
                    }
                    if (temp == null) {
                        temp = forTimeZone(TimeZone.getDefault());
                    }
                } catch (IllegalArgumentException ex) {
                    // ignored
                }
                if (temp == null) {
                    temp = UTC;
                }
                cDefault = zone = temp;
            }
        }
    }
    return zone;
}