When doing quests for new abilities, you may sometimes notice that the enemy you need to kill sometimes won't count for the quest's quota; this is true if your weapon has the enhancement counterattack attached to it.

Enemies killed through counter-attack damage will not be tabulated for your ability quest score. This is not much of a problem if you are fighting common enemies like Flesh Men, but if you have to destroy end-of-act boss enemies, you will want to use a normal SP-using attack instead of relying on counterattacks.