Talking of double negatives, I heard this cautionary tale (possibly on the 'Cat!):

A professor of Languages was giving a lecture on grammar, and got on to the subject of negation. "In some languages, such as English, a double negative is generally interpreted as an affirmative. For example, 'We don't have no bananas' is identical to 'We have bananas'. In some languages -- and some English-speaking sub-cultures, of course! -- a double negative is interpreted as a negative: 'We ain't got no bananas', for instance, clearly means 'We have no bananas'."
[A few muffled laughs]
Encouraged by the possibility of a little professorial humour, he added, "However, as far as I am aware there is no language in which a double affirmative can be interpreted as a negative!".
From the back of the lecture theatre: "Yeah, yeah!"
[Collapse of academic party]

Steve