More or less curious, I encountered this problem last night.
Upon killing Ragnaros 10N for the Heart of Flame, and proceeding to swim in the lava with 3 healers on me to loot the heart, the item that spawned suddenly disappeared when I clicked on it, and I received no credit for it.
No, I did not die at any point during the encounter. No I did not die in the lava afterwards. No I did not move after clicking it. No I did not double click it. It just vanished when I clicked once.
A friend of mine said all I had to do was submit a ticket, saying he seen it happen before or something. So I submitted a ticket. I got a response saying 'sorry for not catching you online (even though I was online), but the GM department cannot do anything about this'.
Seemed to me like the GM was just being a douche, so I followed up the ticket, got another response saying there was no way to prove it dropped.
Even though... I saw it there... and I clicked on it, gear cursor and everything... and it vanished with no credit...?
Would there not be a spawn event in their server logs?
I don't understand why they couldn't at least reset Ragnaros. Sigh.