A mondegreen is where a phrase is misheard and interpreted as something which sounds more or less the same, but is actually quite different from what was actually said.
For example, a speaker might say "What's that toy left on the chair?" and a listener think they said "What's that toilet on the chair?" This is a real example that occurred today, I haven't made it up.
This kind of linguistic error was first called a mondegreen by Sylvia Wright, who wrote that as a child she heard the first verse of a Scots ballad as
Ye Highlands and ye Lowlands,
Oh, where hae ye been?
They hae slain the Earl o' Moray,
And Lady Mondegreen.
Only later did she realize that there was no lady Modegreen. The Earl was the sole victim, and they laid him on the green.
Well-know mondegreens include
"Gladly the cross-eyed bear" for "Gladly the cross I'd bear".
"Good Mrs Murphy" for "Goodness and mercy".
"Me ears are alight" for "The Israelite".
One that a friend told me was the "Potato clock". Whatever is a potato clock? Well, "We need to get a potato clock". Better set the alarm then!
Mondegreens have attracted the attention of psychologists. In The Language Instinct Steve Pinker pointed out that the interesting fact that what the listener hears is often considerably less likely than the intended version. My examples of the toilet on the chair and the potato clock certainly have this property. Pinker interprets this as evidence that we hear what our auditory systems tells us (even though it's an unlikely meaning), not necessarily what makes sense in the context.
I wondered if I could systematically generate some mondegreens and hit upon the idea of reading verse to my dictation software. The latter must try to match sounds to stored words, using some kind of "goodness of fit rule, and I'm pretty sure it will also take into account what words are the most likely ones to follow a given word. I don't think it has a model of the world though, so what it recognizes should fit together plausibly as a word sequence, but might not mean the same as what I actually said.
And sure enough it came up with some modegreens. Here are a couple that amused me. See if you can spot the originals!
"Good thing which this last looked out."
"A poor player structure in French is out upon the stage."