Get Staff of Renewal and Hammer Slam on Rook.

1. Cast Hammer Slam.
2. When there is about 6.5 seconds left on the cooldown use the staff of renewal.
3. As soon as Hammer Slam is cooled down cast it again.
4. As soon as Hammer Slam is cooled down (should be 5 seconds) cast it again.

If timed correctly, the Staff of Renewal buff should end while you are casting the third Hammer Slam.  This causes the cooldown on Hammer Slam to go back up to what it would have been without the Staff of Renewal, ~3 seconds.  Once you finish casting Hammer Slam the cooldown is not correctly reset to 10 seconds so you get a 4th Hammer Slam only a few seconds after the 3rd one.

The behavior for the original 3 Hammer Slams appears to be correct, it's the 4th Hammer Slam that is able to be cast 7 seconds early.

The key to this bug is having the cooldowns of a spell set (staff of renewal buff removal) while you are in the middle of casting that spell.  Instead of starting the cooldown fresh when your spell finishes casting, it remains on whatever the cooldown was set to while you were casting.