Think about that your are at time just at DST time, It moves forward one hour or moves back one hour without living this time portion.
So It is not bug, it is feature.
When you change the timezone which does not use DST (lets say Canada/East-Saskatchewan), you will see what you have expected.
Here is an example.
public static void calendarBug() {
for (int i=0 ; i<6 ; i++) {
Calendar c = Calendar.getInstance();
c.setTimeZone(TimeZone.getTimeZone("Canada/East-Saskatchewan"));
c.clear();
c.set(2015, 2, 27, i, 0);
System.out.println(c.getTime());
}
}
Fri Mar 27 08:00:00 EET 2015
Fri Mar 27 09:00:00 EET 2015
Fri Mar 27 10:00:00 EET 2015
Fri Mar 27 11:00:00 EET 2015
Fri Mar 27 12:00:00 EET 2015
Fri Mar 27 13:00:00 EET 2015