I literally just figured this one out.
The Beatles, “Back in the USSR”. I always heard “[Something] weighted paperback was on my knee. Man, I had a dreadful flight.”
I just figured out the real line is “On the way the paper bag was on my knee. Man, I had a dreadful flight.” I assume the “paper bag” is the airsickness bag, which completely explains why it was such a dreadful flight. Those lines make so much more sense now!
Gloria
I think they got your number.
I think they got the area code
That you’ve been living under.
It’s really “alias,” but all phone numbers have an area code, so the misheard lyric makes some sense.
When I find myself in times of trouble, Mother Mary comes to me
Speaking words of wisdom, Letter B
…
Letter B, Letter B, Letter B, Letter B
Whisper words of wisdom, Letter B
What I’ve always heard…
My sister looks cute in her bracelets and boots
A handful of breeze in her hair
I was today years old when I finally heard the Righteous Brothers do not sing, “If you’ll be living forever” but “If you believe in forever”. Which makes more sense paired with “then life is just a one night stand.”
Slade:
If you got a crush, don’t beat around the bush.
Me:
If you got a crush, don’t be an octopus.
Yeesh, I should have known that wasn’t right.
Now that you mention it, I heard Don’t be an octopus too.
It kind of makes sense though. Like, an octopus is ‘all arms’. If you’ve got a crush on someone, keep your hands and arms to yourself. Don’t be an octopus. 
I’ve just remembered a thread from several years ago, where someone was debating the lyrics to Down Under (Men At Work). Even after showing him the lyrics as written on Colin Hay’s website (i.e., ‘straight from the horse’s mouth’), he would not be convinced he was wrong.
He ended up being banned over that thread. What a silly hill to choose to die on.
Possibly that poster didn’t trust Colin Hay on the grounds that he wouldn’t look him straight in the eye.
Dobie Grey’s “Drift Away”: For ages I thought it went “Give me the Beach Boys and free my soul, I want to get lost in your rock and roll and drift away.”
The actual line is “Give me the beat, boys…”
I’d always imagined the singer drifting away in thought while listening to a Beach Boys record.
I must admit, that’s exactly what I thought too. I like it better with the Beach Boys reference!
Tommy James and the Shondells
For literally decades I thought it was Hangin’ Around, this morning I found out it’s Draggin’ a Line.
I thought it was “…blowing through the chasm in my mind.”
Pfui.
An old mondegreen from when I was a wee one:
“Goodbye, Ruby Tuesday,
Who could hang a nail on you.”
I always thought it was “Hitchin’ a ride”.
Another old personal (and no one else’s!) mondegreen…
from “Penny Lane” - There, beneath the blue, ‘so purple’ skies. (instead of 'suburban’)
I grew up thinking Stevie Nicks was singing, “Just like a one winged dove. Sings a song sounds like she’s singing.”
The smell of fat chicks just put my spine out of place.