Sometimes, our brains race ahead of our ability to communicate with human language, and words get switched around in a sentence. It is something akin to the order of letters in words being inverted, yet they are legible if the first and last letters are in place. Many such errors come from typing errors rather than ignorance of correct spelling. So it is with the spoonerism.
My wife often teases me because I often commit such errors in speech, like “Je vais mettre du poèle dans la pétrole” – I’ll put some stove in the heating fuel. The nonsense of such a sentence make make any reasoning person conclude that words had been switched.
Dr Spooner was a legend in Oxford in his day. The list of examples in the Wikipedia page doesn’t include, or dumbed down, the most outrageous, as he stood with his glass of champagne in his hand to drink to Queen Victoria’s health:
Let us glaze our arses and roast the queer old Dean, instead of Let us raise our glasses and toast the dear old Queen!
Here are the others from the Wikipedia article:
- “Three cheers for our queer old dean!” (rather than “dear old queen,” which is a reference to Queen Victoria)
- “Is it kisstomary to cuss the bride?” (as opposed to “customary to kiss”)
- “The Lord is a shoving leopard.” (instead of “a loving shepherd”)
- “A blushing crow.” (“crushing blow”)
- “A well-boiled icicle” (“well-oiled bicycle”)
- “You were fighting a liar in the quadrangle.” (“lighting a fire”)
- “Is the bean dizzy?” (“Dean busy”)
- “Someone is occupewing my pie. Please sew me to another sheet.” (“Someone is occupying my pew. Please show me to another seat.”)
- “You have hissed all my mystery lectures. You have tasted a whole worm. Please leave Oxford on the next town drain.” (“You have missed all my history lectures. You have wasted a whole term. Please leave Oxford on the next down train.”)
Another interesting phenomenon in England is (or has been) Cockney rhyming slang, a kind of code for thieves and swindlers in the low places of the East End. Right mate, we’ll go round the Jonny Orner, down the frog, up the apples, and ‘ave a butchers at the Tower Bridge.
Jonny Orner = corner
Frog and toad = road
Apples and pears = stairs
Butcher’s hook = look
The Blue Flower 
Of course, many are apocryphal, and Dr Spooner was himself very frustrated by the erroneous ascription of sayings to him. Often, his errors were more conceptual (and more startling), such as the time he spotted a former student in the High, and said to him ‘Tell me, x, was it you or your brother that was killed in the Great War’.
Indeed it can be very amusing to other people. When we do that kind of thing, like stammering, the thing to do is slow down and think before saying something. It is frustrating to have to adapt to the slowness of converting thoughts into spoken words, but that’s what we have to do if we want to avoid ridicule. Perhaps Spooner might have suffered from Alzheimer’s as he got old. Another possibility is trying to multi-task and processing too much information at the same time. We come out as absent-minded because the excessive information becomes irrational. It can also depend on the neurological make-up of a person, for example if he is on the high-functioning end of the autism spectrum (Asperger’s Syndrome). “Aspies” are terrible for lacking tact and thoughtfulness. They say something innocent, and it is interpreted as something bad by another person. This often happened in my childhood when trying to explain things to schoolmasters, which would sometimes merit a whacking!
I suspect Dr Spooner had something like Asperger’s. Clearly it was something which resulted in a confusion between thought and speech. He was, of course, a highly intelligent man.
Yogi Berra, who died last year, also used to get very frustrated that things he hadn’t said were ascribed to him. He did come out with some good ones, though.
‘Good Moaning’ The Spoonerisms remind me of Officer Crabtree , the English spy posing as a Gendarme in ‘Allo Allo’
Then there’s the sermon about how the strains and difficulties of life finally end, for the Christian, in a blessed rest — in which a long and florid description was made (by way of illustration) of the workingman who labors all the day, painfully, to the point of exhaustion until at last, at the setting of the sun, he wends his weary way to his humble home and enters gratefully in, there at last, in great relief, to recline upon his beery wench. (“weary bench”)
Trouble and strife = wife