It was 1988 and the then-vice president, George H. W. Bush, was on a routine visit to Idaho. He was supposed to give a dry speech on agricultural policy and praise his successes alongside President Reagan, live on television. Then he said: “We’ve had triumphs. Made some mistakes. We’ve had some sex... uh... setbacks.”

Long after his political career is consigned to distant memory, President George Bush Senior will be celebrated for this legendary gaffe.

Ah, the Freudian slip. There are the things you want to say, the things you could get away with saying and the things it would be utterly disastrous to utter – which, invariably, are what actually comes out of your mouth. It’s the greatest fear of any public speaker. But what really causes these errors? And do they have any hidden meaning?

For Sigmund Freud, the founder of psychoanalysis, it wasn’t enough to simply ask his patients what they thought. Their true desires, he believed, could only be examined by paying attention to ‘slips of the tongue’ and other clues from the unconscious. A classic slip is, as the saying goes, when you say one thing and mean your mother.

Otherwise known as parapraxis, these verbal stumblings could reveal forbidden urges – such as sex and swearing – which were usually locked safely within the unconscious mind. Verbal errors aren’t random at all, but puzzles to be decoded.

There’s just one problem: Freudian slips, as with many of his other ideas, are extremely difficult to test. Freud may be as famous as Darwin, but many modern-day psychologists, linguists and neuroscientists think that he was wrong about almost everything. But was he wrong about this?