You can't eat your cake and have it too.

William Safire in his NYT column on June 12, 1996:

“Correct usage of a much-abused proverb first recorded in the 16th century has become evidence. In paragraph 185 of his 35,000-word “manifesto,” published under duress by The Washington Post and The New York Times, the Unabomber wrote, “As for the negative consequences of eliminating industrial society — well, you can’t eat your cake and have it too — to gain one thing you have to sacrifice another.” In a letter discovered in Kaczynski’s mother’s home — a letter that inexplicably found its way into the media — the same proverb appears in the same words, with the same lack of a comma before the “too.”

“In both instances, the having and the eating were in correct order. Many people err in saying, “You can’t have your cake and eat it, too,” because you can — first you have it, and then you eat it. The impossible is the other way around; to “eat your cake and have it” is the absurdity that makes the point. Both the Unabomber’s creed and the Kaczynski letter had it right, which is more than can be said for half the quoters of the proverb.”