I was play testing the combat log in order to understand the X+Y=Z formula by making two party members attacking themselves and surprisingly found out that a green ioun stone on my Kensaï protected him from receiving a critical hit.
Behavior Seen: Critical hit prevented from wearing a ioun stone
Expected Correction Indicate the critical hit protection.
Behavior Seen: Combat log is not taking into account the THACO bonus of this stone (rules implementation does take into account of it)
Expected Correction: Take into account the bonus in the Y variable.
[Edit] After some researches, it seems to be a correct behavior. A precision about critical hit immunity in the text explanation would be nice.
[Edit 2] I found a bug in the combat log, the +1 thaco bonus is not taken into account (only in the text) For instance i've got the following log:
[without the stone] 19 + 4 = 23 : Miss [with the stone] 19+4 = 23 : Hit