I'd like to mention that this aspect of the game — not being able to use a creature's ability because of a mistake or mis-click — brought me to these forums.
I'll support the game designer's decisions to keep this aspect of the game, however as a new player, and probably for everyone who encounters this issue, it really does seem like a bug. Please consider making a note of this within the game so people will understand that this was intentional.
On a related note, I'd like to highly encourage the designer/development team to consider allowing you to cancel mistaken clicks like these without a consequence. If there must be a cost to this mistake, at the most just burn the mana cost, but not the ability cooldown (accidentally spending one turn worth of an ability can be HUGE during a match and it's rather harsh punishment for just clicking, in my opinion).
I realize the game must maintain a certain level of difficulty to remain challenging and interesting for the player, but the "creature ability mis-click" is simply frustrating, especially if it's an honest mistake (For instance, "Oops, I meant to click on my Otyugh not my Anubis right now.... crap I wanted to make creature X in my hand invincible this turn.")
Finally, this aspect seems rather contradictory to being able to cancel spells in mid-cast without a consequence, and yet creature abilities become the exception. This also reinforces the player's notion that this is actually a bug.
I'm quite passionate about this suggestion because I believe changing this (or making note of it somewhere) would only improve the interactivity of the game. I really enjoy this game, and I'm only trying to help polish it! Thank you.
