I noticed this in my last campaign mission (battle with the 7 trog wizards at the end of "Beneath the Frozen Earth"): when selecting a target for ranged attacks (shock or zap, I can't recall), certain enemies would display "Unable to Block" (or possibly "Unable to Defend"... I can't recall the exact language) when hovered over with the mouse. Naturally, I selected one such enemy to make sure my attack went through... and the enemy rolled an armor save anyway. I didn't believe this tag was specific to blocks; I have observed enemies that display this tag have not gotten armor saves in the past, when I have known them to hold an armor card in hand. I'm also quite sure that I clicked on the correct target, and not an adjacent one that didn't display this tag. Whichever way the effect is supposed to perform, it does not appear to be consistent.
Armor isn't the same as block. Armor (grey cards) is always rolled and soaks a determined amount of damage, block (green) blocks the full damage but only works if the blocker faces his attacker. (also - penetrating attacks don't trigger armor)
Pengw1n is correct... though I still think there may be a bug where "Can't Block" is displaying when the opponent doesn't have a block to play (I suspect that various weak armors that only work in the same facings as a block anyway are to blame), this particular example is not a bug. My mistake!
I believe the "Can't Block" shows up any time an enemy is facing away from you, even if their hand is empty.
