English is a funny old language. Not content with having the widest vocabulary of any language, we confuse Johnny Foreigner further by having some words with contradictory meanings.
The canonical example is the verb “to cleave”, which can mean “to adhere firmly and closely or loyally and unwaveringly” as in “Therefore shall a man leave his father and mother, and shall cleave unto his wife”. But it can also mean “to split apart”, as in the modern English “cloven hoof”.
Another example is the adjective “quite”. “Quite nice” damns with faint praise, whereas “it was quite beautiful!” intensifies just how beautiful the speaker believes the object to be (although this latter use is probably archaic or posh).
[Added 12 Jan 2013: A tweet from Dan Zambonini reveals that the official term for words that also mean their opposite is "auto-antonym" and there are loads of them.]
A similar contradiction happens with some words which are apparently synonyms. If I tell my wife after a mere 16 hours of choosing dresses and applying cosmetics that “you look a vision”, I am correct. If I tell her “you look a sight”, I’m in trouble. Foreign gentlemen courting English women may wish to ensure they aren’t being misunderstood by pre-emptively answering the inevitable “does my bum look big in this?” appropriately: “You look a vision, and in that frock your bum doesn’t look big at all. Honestly.” is a sure-fire way to an idyllic evening.
Other accidental antonyms can occur by use or omission of the definite article. For example, if Mr Darcy says “Miss Bennet, this chicken tikka masala that you have prepared is bollocks”, he is likely to receive a swift kick in his northanger abbies, because it suggests that the food is substandard (“bollocks” means “testicles”). However, were he to exclaim “Miss Bennet, this chicken tikka masala that you have prepared is the bollocks” (note the definite article, my emphasis), he’s practically guaranteed a bout of boudoir moist cotillion, because “the bollocks” is short for “the dog’s bollocks”, which is approbation, derived from “the mutt’s nuts”, a jovial synonym for “cat’s whiskers” (or “bee’s knees”) — high praise indeed.