Having it Both Ways

English has as number of words, variously called contradictanyms, auto-antonyms, antagonyms, and Janus words, which can mean both one thing and its opposite.  For example, qualified can mean both “competent” and “limited”—as in “Although qualified for the job, he was only a qualified success.” (Janus was the Roman god of beginnings and endings, who looked in two directions.)

There are various reasons that such words occur—development of similar words from different roots (such as cleave, meaning “to adhere” from Old English clifian and "to separate" from Old English cleofan), change of meaning in a word over time (critical, meaning both "vital" and "censorious"), application of an existing word to a new meaning (screen, meaning "to hide from view" and also "to show [a film]"), and plain old ignorance (using literally to mean "virtually" instead of "actually").

However they come about, contradictanyms can make for confusion if you don’t read carefully.  Consider the following sentences;

            Bolt the door or the prisoner will bolt.

            His critical comments were critical to our success.

            Stars are out when the lights are out.

            There was an unfortunate oversight in her oversight of the work.

            He tried to move fast, but he was stuck fast.

            One man was left, but he left.

            If the lights goes off, the alarm goes off.

            I’m in a fix and have no fix for it.

            I’d like to rent your car.  Will you rent it?

The Bard of Buffalo Bayou is something of a contradictanym himself; he thinks what he writes is poetry, but it’s actually the opposite, to wit:

            I shot an arrow into the air,

            It fell to earth I knew not where.

            A man rushed up and yelled at me:
            “You fool—you shot me in the knee!”