Build: 9 122
Steps to reproduce:
1. Construct a new formatter with a null locale
2. Format a Calendar and then check the result.

Actual result:
date/time is formatted to unreadable string. such as \u940f\ufffd

Expected result:
the result should be date/time string, Tuesday or May, etc.

Sample code:
//construct a new formatter
Formatter f = new Formatter(new StringBuilder(), (Locale)null);
//create a new calendar
Calendar c1 = new GregorianCalendar(new SimpleTimeZone(-8 * 60 * 60 * 1000, TimeZone.getAvailableIDs(-8 * 60 * 60 * 1000)[0]), Locale.US);
//set the date/time for the calendar
c1.set(1995, 4, 23, 19, 48, 34);
//format the calendar
f.format("%ta", c1);
//Expected result should be Tue, now the result is \u940f\ufffd
System.out.println(f.toString());

Additional:
This issue doesn't reproduce in JDK 8u76, formatter returns "Tue" as expected.