I encountered this problem yesterday morning, whilst grinding against the Hoppreys. Since Piercing Serpent was at level 7, I thought I'd try to speed it up, and AoE two or three of them at once. Well, it didn't work. When I tried to use the skill, my client crashed, and wouldn't work at all. I reinstalled it and didn't try again. I'll try again now, and see if the reinstalled client has the same problem.

EDIT: Reinstall fixed it. Silly buggy V11 Client. You can close/delete this topic now ^^