From this log:
http://www.worldoflogs.com/reports/rt-ryzaw2u75zpixz5w/dashboard/?s=2330&e=2779

And this topic:

<a href="http://us.battle.net/wow/en/forum/topic/2483689303">http://us.battle.net/wow/en/forum/topic/2483689303</a>

This is the part in question:
[00:55:48.039] Beradon gains Savage Defense from Beradon (Remaining: 18267)
[00:55:54.475] Beradon's Savage Defense is refreshed by Beradon (Remaining: 15009)
[00:55:56.929] Atramedes hits Beradon 54922 (A: 21286)

So you gain SD, then refresh it and lose 3k in shield after 6 seconds. My only guess here is that vengeance decay kicked in bad, but it's not good that a bigger shield can be refreshed.

There are other bigger issues in that thread, but I'm not sure that those are combat log bugs or actual bugs. Investigating them would be good, however.

Edited by Fellhoof on 5/5/2011 7:15 PM PDT