I don't have screenshots, because it happened over several turns, but I was playing against an arena deck with Unstable Gases. It had one in play for awhile that it wasn't using. I was wondering about this, since I was around 30-40hp, but reasoned that it didn't want to kill its own dragonflies with it. (It had 2 or 3, plus a flying eagle's eye) It had two burning pillars, but kept spending the quanta every turn on explosions or other things.
A couple turns later it played a second Unstable Gas, and still didn't use the first one. This surprised me, but I figured it would just use them both next turn to deliver the coup de grace.
The next turn, it had 2 fire quanta from the pillars, two Unstable Gases, and my hp was in the 20s somewhere, I think 22. It didn't use either gas, preferring to cast explosion on my weapon instead of killing me with the two gases.
The turn after that, it wastes the fire quanta on another explosion, casts Sky Blitz, which due to my Dusk shield, the dragonflies and flying eagle's eyes, doesn't kill me. (I can't fault it for sky blitzing there, but it should have just killed me with gases)
The turn after that, it doesn't even play any fire cards, it just ignores the gases despite having two unused fire quanta, attacks, and gets through my dusk shield, finally killing me.
I'm wondering if maybe some card I had in play made the AI erroneously think I could reflect its gases. Unfortunately, other than the improved dusk shield, I can't remember what permanents I had. I think I had a sanctuary and an hourglass and the shield left at the end, and some towers; everything else was blown up.
Short version: If the AI has enough unstable gases and enough fire quanta to kill the player, it should just do it! :p