by Darvin on Mon Nov 18, 2013 6:53 pm
I believe this is an intentional feature to save you from a grizzly death. If you were to fail to notice that you lacked the mana for fireball then clicked on the target, you'd attack it and possibly get yourself killed. The game "jams up" to keep you from making possible mistake. Perhaps it could be more graceful about it, but in principle it makes sense.
Keep in mind that if you're using hotkeys you've only got about a tenth of a second to notice that the fireball failed to activate before you reflexively click, so this can be a huge life-saver in those circumstances.