Ooh! Ooh! I can play this one - I've folded kings three times that I remember in low stakes NLHE cash games, and each time I was shown aces.
I only remember doing it once in a tournament.
Early in the SatP main event this past April, I was playing a hand against
@BootySnatcher and everybody was still very deep. IIRC he opened UTG, player in MP raised, I 4-bet, and
@BootySnatcher 5-bet shoved into both of us. After the other player snap-folded, I tanked for a bit, sighed, and did the same. I showed my kings, and he showed me the aces I knew he had.