So before I say anything else, I want to say that I could be wrong about this. I tried to test it but because of Frostbite’s 15% proc chance it was pretty hard to get anything consistent.
Anyway, this is Frostbite, a Frost Mage talent:
Gives your Chill effects a 15% chance to freeze the target for 5 sec.
It seems that when this effect procs on a target that is currently rooted by Frost Nova, it will instead not proc and you will get an error message saying “A more powerful effect is already active” and Frostbite will not apply.
Again, this was really hard to test and I’m still not 100% sure on it. Frostbite only has a 15% proc chance and I had to land a spell that would have procced Frostbite, but only on a target with Frost Nova applied in order to test it. This was really hard to do because Frost Nova only lasts a few seconds and it tends to break on damage a lot.
I dont know if this is an intended effect or not, but I seem to remember being able to Frost Nova someone into Frostbite rooting them for several seconds back in the day.