Upsetting Vengeance (Combat)Source Armor Master's Handbook pg. 13
You punish those who fail to press their attack.
Prerequisites: Dex 13, Combat Reflexes, Improved Shield Bash, Upsetting Shield Style, Upsetting Strike, proficiency with bucklers.
Benefit: While using Upsetting Shield Style, the penalty on attack rolls foes take for being hit by your shield bash attack applies to their attacks against any target. If an adjacent foe taking this penalty makes an attack roll against any target and misses the target’s AC by 5 or more, that foe provokes an attack of opportunity from you. You can only make one attack of opportunity against an opponent as a result of this feat or Upsetting Strike each round.