A song is Ruined Forever when you become aware of a very slight change in a word or phrase or even emphasis that completely changes the song. Sometimes somebody does this for you. Sometimes your own brain does it, unbidden.
For example: The song "El Paso" contains the line "Blacker than night were the eyes of Felina." More than thirty years ago somebody sang to me "Blackened with lice were the thighs of Felina." Boom! That song is Ruined Forever. What Ruins the song Forever is merely a couple of similarly emphasized vowel sounds in very different words.
The old Chad Mitchell Trio faux-temperance song "Away with Rum" contains the passage:
We never eat fruitcake because it has rum
And one little bite turns a man to a bum.
Oh can you imagine a sorrier sight
Than a man eating fruitcake until he gets tight.
A man-eating fruitcake is a strange and dangerous creature indeed. Look out for the man-eating fruitcake! Aaaggghhh! And it requires changing not a word, to Ruin this song Forever.
This is common in religious songs. In the Sacred Harp there is a slow, lugubrious, powerful minor-key song with this verse:
He dies the friend of sinners dies
Lo, Salem's daughters weep around
How do you Ruin this song Forever? Simply by substituting "sleep" for "weep".
There are scores and scores of songs that are Ruined Forever to me. But this one is the worst:
A friend of mine claims that when she was a little girl watching the Jerry Lewis telethon for Muscular Dystrophy, she thought the big emotional song he sang at the end was not "You'll Never Walk Alone" but "You'll Never Walk Again." Picture it: Jerry Lewis, belting out:
... and you'll NEVER WALK AGAIN
You'll NEEEEE VERRRRRR WAAAAALK AAAAAA GAAAAAIN.
Ruined forever, I tell ya'.