FULL PRODUCT VERSION :
java version "1.5.0_03"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_03-b07)
Java HotSpot(TM) Client VM (build 1.5.0_03-b07, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
DateFormat.parse() has an undocumented side-effect: calling it with a string that contains a timezone, different from one that was explicitly set, will overwrite the DateFormat's internal timezone with that of the string.

Further attempts to use the DateFormat for formatting will use the new timezone.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the attached test case.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Output of the test should be two identical GMT TimeZones, like:

sun.util.calendar.ZoneInfo[id="GMT",offset=0,dstSavings=0,useDaylight=false,transitions=0,lastRule=null]
sun.util.calendar.ZoneInfo[id="GMT",offset=0,dstSavings=0,useDaylight=false,transitions=0,lastRule=null]

ACTUAL -
Output of the test is:

sun.util.calendar.ZoneInfo[id="GMT",offset=0,dstSavings=0,useDaylight=false,transitions=0,lastRule=null]
sun.util.calendar.ZoneInfo[id="America/Los_Angeles",offset=-28800000,dstSavings=3600000,useDaylight=true,transitions=185,lastRule=java.util.SimpleTimeZone[id=America/Los_Angeles,offset=-28800000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=3,startMonth=3,startDay=1,startDayOfWeek=1,startTime=7200000,startTimeMode=0,endMode=2,endMonth=9,endDay=-1,endDayOfWeek=1,endTime=7200000,endTimeMode=0]]


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.text.DateFormat;
import java.util.TimeZone;

public class TestDateFormat {
    public static void main( String[] args ) throws Exception {
        DateFormat f = DateFormat.getDateTimeInstance( DateFormat.SHORT,
                DateFormat.FULL );
        f.setTimeZone( TimeZone.getTimeZone( "GMT" ) );
        System.out.println( f.getTimeZone() );
        f.parse( "6/15/05 7:00:00 AM PDT" );
        System.out.println( f.getTimeZone() );
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Use separate DateFormats for formatting and parsing.