Patch to ensure that Batcache never returns a negative max-age which occurs if the request includes an If-Modified-Since header that is not equal to the cached response's Last-Modified time.

advanced-cache.php.patch

diff --git a/wp-content/advanced-cache.php b/wp-content/advanced-cache.php
index 83f24ee..4d01cd1 100644
--- a/wp-content/advanced-cache.php
+++ b/wp-content/advanced-cache.php
@@ -338,7 +338,7 @@ if ( isset($batcache->cache['time']) && ! $batcache->genlock && time() < $batcac
 	// Use the batcache save time for Last-Modified so we can issue "304 Not Modified"
 	if ( $batcache->cache_control ) {
 		header( 'Last-Modified: ' . gmdate( 'D, d M Y H:i:s', $batcache->cache['time'] ) . ' GMT', true );
-		header('Cache-Control: max-age=' . ($batcache->max_age - time() + $batcache->cache['time']) . ', must-revalidate', true);
+		header( 'Cache-Control: max-age=' . max(0, $batcache->max_age - time() + $batcache->cache['time']) . ', must-revalidate', true );
 	}
 
 	// Add some debug info just before </head>

See full writeup at http://wordpress.org/support/topic/if-modified-since-request-header-can-cause-a-cache-control-negative-max-age?replies=1