By Slink, via Geoff:

The attached patch is a one-liner for waiter/cache_waiter_ports.c,
the code was referencing sp->t_open where it should have been
sp->t_idle, which was causing assertion failures on idle session
timeouts.

bin/varnishd/waiter/cache_waiter_ports.c

@@ -3,7 +3,7 @@
  * Copyright (c) 2006 Varnish Software AS
  * Copyright (c) 2007 OmniTI Computer Consulting, Inc.
  * Copyright (c) 2007 Theo Schlossnagle
- * Copyright (c) 2010 UPLEX, Nils Goroll
+ * Copyright (c) 2010-2012 UPLEX, Nils Goroll
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -220,7 +220,7 @@ vws_thread(void *priv)
 
 		if (sp) {
 			double tmo =
-			    (sp->t_open + cache_param->timeout_idle) - now;
+			    (sp->t_idle + cache_param->timeout_idle) - now;
 
 			/*
 			 * we should have removed all sps whose timeout