I wanted to rush the fight with the dragon, so decided to just throw off all those poison potions (collected during my journey) at once. In the same time, i frose time non-stop with the green boxes (which here too i didn't use at all ).
Anyway, at one point, i cast some fireballs too, and then this happended :
it's an animated gif, hope it's ok

Maybe i throw too much at one time

It's just something i have never seen in this game (poison clouds & fireball that never end). I then walked in the never-ending-spell, and got an error from the game.
Ah well, i guess next time i will be more respectful in my approach..