Fixed since 1.0.rc15:

http://dovecot.org/list/dovecot-news/2006-November/000023.html

Index: src/lib/file-cache.c
===================================================================
RCS file: /var/lib/cvs/dovecot/src/lib/file-cache.c,v
retrieving revision 1.12.2.1
retrieving revision 1.12.2.4
diff -u -r1.12.2.1 -r1.12.2.4
--- src/lib/file-cache.c	28 May 2006 23:43:44 -0000	1.12.2.1
+++ src/lib/file-cache.c	18 Nov 2006 23:35:35 -0000	1.12.2.4
@@ -128,8 +128,8 @@
 	i_assert(psize > 0);
 
 	bits = buffer_get_space_unsafe(cache->page_bitmask, 0,
-				       poffset / CHAR_BIT +
-				       (psize + CHAR_BIT - 1) / CHAR_BIT);
+				       (poffset + psize + CHAR_BIT - 1) /
+				       CHAR_BIT);
 
 	dest_offset = poffset * page_size;
 	dest = PTR_OFFSET(cache->mmap_base, dest_offset);
@@ -282,7 +282,7 @@
 	}
 
 	bits = buffer_get_space_unsafe(cache->page_bitmask, offset / CHAR_BIT,
-				       (size + CHAR_BIT - 1) / CHAR_BIT);
+				       1 + (size + CHAR_BIT - 1) / CHAR_BIT);
 
 	/* set the first byte */
 	for (i = offset % CHAR_BIT, mask = 0; i < CHAR_BIT && size > 0; i++) {