It was in the spring of 2006. I was having a three-hour session with Mistress Kassandra (who has since retired from being a professional Domme).
She had me tied down to a bondage table with my arms extended far above my head and a big, thick pillow under my butt. She climbed up on the table, straddling me, and dug into my ribs. Using both thumbs, she did deep tickling up and down my rib cage, then went to my most vulnerable spot, between my second and third ribs (counting from the bottom up).
Staying on that spot, on both sides simultaneously, she smiled as I went berserk, thrashing and struggling to no avail. After an endless time of ticklish agony, she looked into my eyes, deeply, and, without stopping the relentless movement of her thumbs, she said, "I know that you're suffering and it amuses me. It amuses me greatly."
That broke me the most ever in all my experiences, before and since. God bless you, Mistress Kassandra.