I threw on passively crush immune gear during the trash, to test that. Without putting up shield block, I was regularly taking unblocked hits. The first night it came up I kinda scratched my head and tried to figure out if I was being retarded and letting mobs get behind me. But it seems Blizzards has given mobs some demented expertise or something, which reduces their chance to be dodged by 20%. The giant robots are definitely sneaking unblocked attacks through a ~103% block+avoidance set. I've gotten unblocked hit - parry - unblocked hit sequences more than once in my combat log, so they're not sneaking behind me. I think the numbers are right: entities in Sunwell have a ~20% dodge chance reduction to all attacks.
Wow, that's all kinds of annoying.
-Splug
EDIT: While that dodge chance reduction does seem to apply to the trash as well as the bosses, it's worth noting that nothing in here seems to actually score crushing blows either. So losing crush immunity is not game breaking, but it will cause unblocked hits, which is a ~5-10% increase in damage taken that swing.
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(Usually) Prot warrior:
http://www.wowarmory.com/character-sheet.xml?r=Illidan&n=Splug