Index: src/test/org/apache/lucene/util/packed/TestPackedInts.java
===================================================================
--- src/test/org/apache/lucene/util/packed/TestPackedInts.java	(revision 930109)
+++ src/test/org/apache/lucene/util/packed/TestPackedInts.java	(working copy)
@@ -222,4 +222,11 @@
 
     dir.close();
   }
+
+  public void testMutableBug() throws Exception {
+    PackedInts.Mutable mutable = PackedInts.getMutable(26, 5);
+    mutable.set(24, 31);
+    mutable.set(4, 16);
+    assertEquals("The value #24 should remain unchanged", 31, mutable.get(24));
+  }
 }
Index: src/java/org/apache/lucene/util/packed/Packed64.java
===================================================================
--- src/java/org/apache/lucene/util/packed/Packed64.java	(revision 930109)
+++ src/java/org/apache/lucene/util/packed/Packed64.java	(working copy)
@@ -78,22 +78,26 @@
    * The setter requires more masking than the getter.
   */
   private static final long[][] WRITE_MASKS =
-          new long[ENTRY_SIZE][ENTRY_SIZE * FAC_BITPOS];
+    new long[ENTRY_SIZE][ENTRY_SIZE * FAC_BITPOS];
   static {
-      for (int elementBits = 1 ; elementBits <= BLOCK_SIZE ; elementBits++) {
-          long elementPosMask = ~(~0L << elementBits);
-          int[] currentShifts = SHIFTS[elementBits];
-          long[] currentMasks = WRITE_MASKS[elementBits];
-          for (int bitPos = 0 ; bitPos < BLOCK_SIZE ; bitPos++) {
-              int base = bitPos * FAC_BITPOS;
-              currentMasks[base  ] =~((elementPosMask
+    for (int elementBits = 1 ; elementBits <= BLOCK_SIZE ; elementBits++) {
+      long elementPosMask = ~(~0L << elementBits);
+      int[] currentShifts = SHIFTS[elementBits];
+      long[] currentMasks = WRITE_MASKS[elementBits];
+      for (int bitPos = 0 ; bitPos < BLOCK_SIZE ; bitPos++) {
+        int base = bitPos * FAC_BITPOS;
+        currentMasks[base  ] =~((elementPosMask
                                  << currentShifts[base + 1])
                                 >>> currentShifts[base]);
-              currentMasks[base+1] = ~(elementPosMask
-                                 << currentShifts[base + 2]);
-              currentMasks[base+2] = currentShifts[base + 2] == 0 ? 0 : ~0;
-          }
+        currentMasks[base+1] =
+          ~(elementPosMask << currentShifts[base + 2]);
+        currentMasks[base+2] = currentShifts[base + 2] == 0 ? 0 : ~0;
+        if (bitPos <= BLOCK_SIZE - elementBits) { // Second block not used
+          currentMasks[base+1] = ~0; // Keep all bits
+          currentMasks[base+2] = 0;  // Or with 0
+        }
       }
+    }
   }
 
   /* The bits */
Index: src/java/org/apache/lucene/util/packed/Packed32.java
===================================================================
--- src/java/org/apache/lucene/util/packed/Packed32.java	(revision 930109)
+++ src/java/org/apache/lucene/util/packed/Packed32.java	(working copy)
@@ -77,7 +77,7 @@
    * The setter requires more masking than the getter.
   */
   private static final int[][] WRITE_MASKS =
-          new int[ENTRY_SIZE][ENTRY_SIZE * FAC_BITPOS];
+    new int[ENTRY_SIZE][ENTRY_SIZE * FAC_BITPOS];
   static {
     for (int elementBits = 1 ; elementBits <= BLOCK_SIZE ; elementBits++) {
       int elementPosMask = ~(~0 << elementBits);
@@ -86,11 +86,15 @@
       for (int bitPos = 0 ; bitPos < BLOCK_SIZE ; bitPos++) {
         int base = bitPos * FAC_BITPOS;
         currentMasks[base  ] =~((elementPosMask
-                << currentShifts[base + 1])
-                >>> currentShifts[base]);
+                                 << currentShifts[base + 1])
+                                >>> currentShifts[base]);
         currentMasks[base+1] = ~(elementPosMask
-                << currentShifts[base + 2]);
+                                 << currentShifts[base + 2]);
         currentMasks[base+2] = currentShifts[base + 2] == 0 ? 0 : ~0;
+        if (bitPos <= BLOCK_SIZE - elementBits) { // Second block not used
+          currentMasks[base+1] = ~0; // Keep all bits
+          currentMasks[base+2] = 0;  // Or with 0
+        }
       }
     }
   }