Hello, hope you're having fun playing DBOG!
Well, I don't know If I translated the title right, anyway, I'll tell you why does it happen.
When you give "Negative" Buff to the enemy and you instantaly do a basic attack right after pressing the key to cast that buff and you try to do an offensive skill while you're attacking(basic) you'll get this error, making you be unable to hit the enemy for some seconds.
Just like how did pass to me: