I think I found a bug

Forgot to take a screenshot, but I added C'Thun, the Shattered to my deck, and got the Small Pouches treasure. It pulled C'Thun into my hand at the start of the game, and as a result his "start of game" effect never triggered - I just had him in hand.

I supposed its not really a bug, as much as it is an unintended effect of how the card works. The way it works is that cthun will never be offered in the mulligan phase, and will break at the start of the match. The effect of the 'start of the match' takes place according to the placement of the cards in the deck, so if you happen to put both Genn Greymane and cthun in an even deck, and cthun is shuffled on top of genn, then cthun will break first and genn's effect will never trigger (because cthun's cards are odd)

So in this case it could be that the game sets passives to function on priority, so you just got lucky having your cthun within range of the draw.

The idea that order matters for various "start of game" effects is all well and good, but those "start of game" effects ought to trigger whether or not the card ends up in your opening hand, and in this case it didn't. The pieces of C'Thun never shuffled into my deck, I just got the 10-drop at the start of the game.