--- old/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	2012-07-24 11:52:11.368587268 -0700
+++ new/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	2012-07-24 11:52:11.147754834 -0700
@@ -3580,7 +3580,11 @@
   DirtyCardQueueSet& dcqs = JavaThread::dirty_card_queue_set();
   size_t buffer_size = dcqs.buffer_size();
   size_t buffer_num = dcqs.completed_buffers_num();
-  return buffer_size * buffer_num + extra_cards;
+
+  // PtrQueueSet::buffer_size() and PtrQueue:size() return sizes
+  // in bytes - not the number of 'entries'. We need to convert
+  // into a number of cards.
+  return (buffer_size * buffer_num + extra_cards) / oopSize;
 }
 
 size_t G1CollectedHeap::max_pending_card_num() {