Index: src/backend/storage/buffer/bufmgr.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/storage/buffer/bufmgr.c,v
retrieving revision 1.193
diff -c -c -r1.193 bufmgr.c
*** src/backend/storage/buffer/bufmgr.c	12 Aug 2005 05:05:50 -0000	1.193
--- src/backend/storage/buffer/bufmgr.c	12 Aug 2005 21:40:41 -0000
***************
*** 153,158 ****
--- 153,160 ----
  		 * block is not currently in memory.
  		 */
  		bufHdr = BufferAlloc(reln, blockNum, &found);
+ 		/* we are guaranted that nobody else has touched this will-be-new block */
+ 		Assert(!(found && isExtend));
  		if (found)
  			BufferHitCount++;
  	}