If you are aware of some mistake in network design that you have seen more than once, feel free to post it here. Explanation, why you think that it is a mistake would be great.
Double Sentry pattern:

Two Sentries side by side will not make your defense any stronger as there can be only one antivirus on any node. Detailed explanation was given in another thread here: http://forum.tricksterarts.com/viewtopic.php?f=9&t=15