"My stomach hurts."
"It's your own fault. You shouldn't have eaten so much food."
"Well, I could have not eaten that large dinner my mother-in-law set before me, but I didn't want to hurt her feelings. Now if you'll excuse me, I'm going to go take an Alka-Seltzer."
This implies that you are unable to eat something.I could not have eaten that
"Why didn't you come to the company picnic?"
"I could not have eaten anything there; I'm an Orthodox Jew and I had no way of telling which foods were Kosher."
"That dessert looked delicious, but I could not have eaten it, since it had almonds in it and I am allergic to nuts."