By this, I mean song lyrics that would not have had alternate interpretations or mondegreens at the time the song was originally written and popular but thanks to changes in culture gain those issues. My best example is The Gambler by Kenny Rogers, which was around for 11 years before one of the lines suddenly sounded like a mention of an iconic portable gaming console. (Also, the song Tiny Dancer came out more than 5 years before the name “Tony Danza” became well known.)
I’m not sure exactly when fax machines started becoming common, but I’m pretty sure it was after the 1980 AC/DC song that starts out: “She was a fax machine…”
Probably every song ever written before 1970 or so that contains the word “gay”. Examples would include “My Old Kentucky Home” (“'Tis summer, the people are gay”), “Jambalaya” (“Pick guitar, fill fruit jar and be gay-o”), and “Have Yourself a Merry Little Christmas” (“Make the Yuletide gay”).
Speaking of the change in “gay”, can I ask one about “making love”?
During my life, “making love” meant the actual sex act. Obviously it didn’t much before 1960. When Mary Hatch yells up to her mom that George Bailey “is making violent love to me” she obviously wasn’t having screaming sex on the living room floor.
So, what DID they mean by that phrase?
From other times I’ve seen the phrase in old books, it apparently meant using words. Stating one’s admiration/love, possibly requesting that the love be reciprocated, etc.
Ol’ George wasn’t physically grabbing a$$, just verbally hitting on Mary.
Yeah, in the old days, to “make love” to someone was to woo them or make advances to them. It is funny to see it turn up in old books and movies.
The Boris Karloff film The Mummy, from 1932, has a two-fer of these lines: A character suggests that the reason the mummy was cursed was because he “got too gay with the vestal virgins.” Later, the heroine says, “Don’t you think I’ve had enough excitement for one evening, without a strange man making love to me?”
Thanks! That always confounded me.
Sex didn’t exist when “making love” was used initially. People would kiss and coo, get married, and then 6 months later a baby would show up in the cabbage patch. Sex got popular in the 60s but people were hesitant to use the word and so they extended the innocent form of “making love” to mean sex. It still worked the same for a while, 6 months after the wedding a baby would show up in the cabbage patch. Finally, people got used to the idea of having sex and the length of pregnancies was extended to 9 months.
“Pitching woo” is another phrase that has different connotations these days.
“By Jove, you’re right!” he ejaculated.
And all of this happened in black-and-white.
Of course! The whole world was in black-and-white up until not long before I was born. 
Uh … no. Fax machines actually predate the telephone. The first telephone fax machine was patented in 1964.
This isn’t really on-topic, but I have to share it. I was at a movie theater to see the 2013 production of Much Ado About Nothing when I heard a teenaged boy say to his girlfriend, “It’s, like, modern Shakespeare, but still in black-and-white.”
Well, of course. They didn’t have color film in Shakespeare’s time. Duh.
The question was when fax machines became common, not when they were invented.
“She was a fax machine” is a mondegreen. The actual lyric is, “She was a fast machine.”
Also, fax machines were common in businesses by 1980, when the song was written. Home faxes came along later.
“She was a fax machine / Which is a mondegreen / Was the best off woo man that I ever seen…” New ear worm! 
In Bullitt (1968) the SFPD gets a copy of Ross’ ID via a handset modem fax. Mind you it took five minutes per page, was as big as a small car, and made more noise that a 707 taking off, but it was a [del]sex[/del] fax machine.
It even turns up in Jane Austen’s “Emma” (published 1816) where Emma is astounded to find a young clergyman, Mr Elton:-
“her attention demanded, and Mr. Elton actually making violent love to her” in a carriage on the way home from a party!