FULL PRODUCT VERSION :
java version "1.5.0_06"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_06-b05)
Java HotSpot(TM) Client VM (build 1.5.0_06-b05, mixed mode, sharing)

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

A DESCRIPTION OF THE PROBLEM :
String.format throws an ArrayOutOfBoundsException with certain formats and BigDecimal values but other formats or values work fine.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
String.format( "%4.0f", new BigDecimal( "99.95" ) )

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
 100

ACTUAL -
java.lang.ArrayIndexOutOfBoundsException
at java.lang.System.arraycopy(Native Method)
at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:519)
at java.lang.StringBuilder.append(StringBuilder.java:190)
at java.util.Formatter$FormatSpecifier$BigDecimalLayout.layout(Formatter.java:3690)
at java.util.Formatter$FormatSpecifier$BigDecimalLayout.<init>(Formatter.java:3610)
at java.util.Formatter$FormatSpecifier.print(Formatter.java:3545)
at java.util.Formatter$FormatSpecifier.print(Formatter.java:3460)
at java.util.Formatter$FormatSpecifier.printFloat(Formatter.java:2716)
at java.util.Formatter$FormatSpecifier.print(Formatter.java:2664)
at java.util.Formatter.format(Formatter.java:2430)
at java.util.Formatter.format(Formatter.java:2364)
at java.lang.String.format(String.java:2520)


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.math.BigDecimal;

public class BigDecimalFormatExceptionDemo {
   public static void main(String[] args) {
      try {
         System.out.println( System.getProperty( "os.name") );
         System.out.println( System.getProperty( "os.version") );
         System.out.println( System.getProperty( "java.version") );
         BigDecimal v1 = new BigDecimal("99.00");

         System.out.println( String.format( "%4.2f", v1) ); // OK
         System.out.println( String.format( "%4.2f", v1.doubleValue() ) ); // OK
         System.out.println( String.format( "%4.0f", v1.doubleValue() ) ); // OK
         System.out.println( String.format( "%4.0f", v1) );

         BigDecimal v2 = new BigDecimal("99.95"); // OK
         System.out.println( String.format( "%4.2f", v2) ); // OK
         System.out.println( String.format( "%4.2f", v2.doubleValue() ) ); // OK
         System.out.println( String.format( "%4.0f", v2.doubleValue() ) ); // OK
         System.out.println( String.format( "%4.0f", v2) ); // Error
      }
      catch ( Throwable x ) {
         x.printStackTrace();
      }
   }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
String.format( "%4.0f", new BigDecimal( "99.95" ) )

generates the error but you can use

String.format( "%4.0f", new BigDecimal( "99.95" ).doubleValue() )

as a workaround.