2011-09-22 : If the defending player controls no creatures when Grimgrin attacks, the last ability will be removed from the stack and have no effect.

2011-09-22 : If the targeted creature is an illegal target by the time Grimgrin's last ability resolves, the entire ability doesn't resolve and none of its effects will occur. You won't put a +1/+1 counter on Grimgrin.

2013-07-01 : If Grimgrin's last ability resolves, but the targeted creature isn't destroyed (perhaps because it regenerated or has indestructible), you'll still put a +1/+1 on Grimgrin.