Sidebar, Part II: According to TWoP Member, Makenna, Eddy Kitsis and Adam Horowitz explain this in the Podcast. Makenna writes: "About the necklace, in the podcast Eddy and Adam mention that by the time Henry travels to the Netherworld the second time the necklace has already been cracked/damaged due to the unforeseen power/danger of the world. So supposedly the necklace was already beginning to crap out when it officially "broke" when Charming crashed thru the floor?"
Sidebar, Part III: I haven't listened to the podcast, so I'm just taking Makenna at her word, but it does nothing to mitigate the situation for me. If the necklace cracked when Henry was wearing it, it was never called to our attention. We never saw him try to use it or acknowledge that he tried, so the whole time I'm watching Henry in the Fiery Room of Dreams, I'm disgusted that he's wearing this ostentatious magical bling and that he had the presence of mind to use it last week, but doesn't seem to try this week. Meta explanations don't cut it, kids. I want to learn the story in the story. Besides which, Charming does manage to leap through the flames, get some space from them and talk to Snow, so I'm just not buying it. Blah.
Sidebar, Part IV: Before she disappeared, Aurora couldn't hear Henry over the flames, but Henry didn't even try to control them. Because they couldn't let Aurora hear him (so that Snow would have to go into the Room of Dreams) and couldn't let Henry return, it would have worked better for me if Henry calmed the flames, Aurora came over to him and then got ripped away (the violence of which could then certainly account for his burn). Or, if back in Storybrooke, instead of "What caused this?" Regina's question to Rumpy had been "Why didn't the necklace protect him?" Rumpy could have said, "It appears cracked, also when you venture into the Netherworld blah blah blah burnt cakes." Okay, I think I've got that off my chest. I don't want to tear down this show. I love it. My point is, make what I am watching matter while I am watching it, please. Thank you. Carry on.