Bright Eyes
I loved Watership Down, and by proxy, rabbits. My brothers delighted (once they realised it made me cry) in following me around, singing 'Bright Eyes, caught on barbed wire'. There were a lot more words. I seem to have suppressed the memory though.
(mookroolz, Thu 27 Jan 2005, 14:04, Reply)

Eastenders Theme Tune
When we were at school, a mate of mine fell out with his mum. His revenge was to change the lyrics to the Eastenders theme tune. Only the fist line. It went:

Anyone can fuck my mum,
She's the biggest slag in Chellllll-aston


Even now I still occasionally find myself saying 'did anyone watch Anyone Can Fuck My Mum last night?'
(jimcrock, Thu 27 Jan 2005, 14:01, Reply)

I think I'll be answering this question several times
I always mishear songs, so I do this quite often.

My first ever was to Silent Night. Not just your ordinary "Round John Virgin" or "Sleep in heavenly peas," though I did that one, too. I messed up the whole first verse.

Silent Night, Wholey night
All is wrong, all is right
Round your herd up, Mother and Child.
Wholey in fact so tender and wild.
Sleep in heavenly peas.
Sleep in heavenly peas.
(dragonrose, Thu 27 Jan 2005, 14:00, Reply)

Ahem:
"Leprosy,
Bits and pieces falling off of of me,

I'm not half the man I used to be,
Oh I believe,
In Leprosy"

(To the tune of Yesterday by the beatles. My mate actually made it up and three of us collapsed in hysterics one school choir practice....*reminisces*)
(Frustrated IT Support Person - now with added network support., Thu 27 Jan 2005, 13:58, Reply)

*ahem*
Reach for the bar
drink every beer pump drier
Reach for the bar
climb every optic higher!
(to the theme of s club 7's 'Reach')

this is sung fairly regularly during our friday night drinking sessions, by a group of about 10 pissheads (us)
( lo, the Greebo Warrior - Est. 2002, Thu 27 Jan 2005, 13:57, Reply)

Freddie Mercury
After Freddie kicked the bucket and they re-released that solo song he did, I often found myself singing for the wordless dee dum . . .part:

Poor old Freddie,
He's deed already . . .
(Barry Subchimp, Thu 27 Jan 2005, 13:55, Reply)

My friends and I were chucked out (in our younger days I might add)
of a pub for filling up the jukebox with money and putting Otis Reading's "Sitting On The Dock Of The Bay" on constantly. . .

Then singing our lyrics over the top.

Dum, dum dumm dum dumm,
Dum dumm dum dumm

Sittin' in the mornin' sun,
I'll be sitting in a barrel of cum.
Watching the shit stabbing,
I won't fuck another gay again.

Sittin' on the cock of a gay,
Watchin' the shite flow away,
Ooooh,
Just sittin' on the cock of a gay,
Wastin' shi-hi-hi-hi-hite.

Looks no gay's gonna change,
Every gay still remains the same.
I can't do what ten homos tell me to do,
So I guess I'll remain the same. . .

Just sittin' here resting my bone,
But these homos just won't leave me alone,
For two thousand miles I roamed,
Just to make this cock my home,

So now I'm jus' gon' sit on the cock of a gay,
Wastin' shi-hi-hi-hi-hite. . .

(Whistle now, to fade)
(Highlander, Thu 27 Jan 2005, 13:53, Reply)

Sick Boy
For that Blue song:
I used to sing: "I aboot dee'd in Aberdeen, I aboot died, in Aberdeen, I aboot died . . .

Glad to know someone else heard it that way.
(Barry Subchimp, Thu 27 Jan 2005, 13:51, Reply)

A bit obscure
Ranting four-eyed Scotch brothers the Proclaimers have a marvellous song called "Oh Jean" that closes their magnum opus "Sunshine on Leith".

Chorus goes "Oh Jean, oh Jean, you let me get lucky with you!"

One line goes "For your soul and body my heart's going to pound, even after I'm dead and I'm laid in the ground".

Me - decided to sing "For your swollen body..." while performing said song at an open mic night. Cue much laughing. Unintentional, but I got a job lot of applause so...brilliant.
(HectorProtector, Thu 27 Jan 2005, 13:47, Reply)

Shishter's friend thought that the "I'm horny" song went
"It's funny, funny funny funny."

What a spesh.
( Naive Amoeba Bored Cig, Thu 27 Jan 2005, 13:47, Reply)

Slipknot song - Spit it out
A lovely loud metal song with lots of possibilities, but mine and the missus faves to sing are...

"Can’t hear shit cuz I keep gettin’ louder" translates into: "don't give a shit, give us the keys to the Lada"

and the final metal chant of:" Fuck me, I'm all out of enemies" is much much better as "Fuck Me! I'm all out of Bread and Meat!"
(Scribble, Thu 27 Jan 2005, 13:47, Reply)

Blue
I always thought that the song Blue went:

I'm blue, if I was green I would die,
If I was green I would die,
If I was green I would die...
(Scynthius, Thu 27 Jan 2005, 13:45, Reply)

I have a habit
of singing along to almost any tune (most often jingles or tv themes) using the words 'F*ck' and 'Your mum' in various iterations.
'Up the bum', and indeed, 'cum' rhyme, which is useful.

For example try singing 'And then you'll find I f*cked your mum' to the cinema bit at the end of the Simpsons credits. Its easy.
(Baj does Minecraft - http://www.youtube.com/w92baj, Thu 27 Jan 2005, 13:44, Reply)

This is obviously going back a few years
But for some reason when getting to the line "I'll reconsider this foolish notion" in Faith by George Michael I always used to sing

"I'll Nova Scotia this Terry Wogan"

I was about 8 at the time :/
(Arnolfini wasn't feeling very funny on, Thu 27 Jan 2005, 13:37, Reply)

There was a somg called 'Blue'
By Eiffel 65 which sounded frighteningly like it was saying:

"Die in Aberdeen, I will die in Aberdeen"

Which was odd as, when I made the connection, we were offered relocation to Aberdeen or redundancy from an old job I was in.
( sick_boy, Thu 27 Jan 2005, 13:37, Reply)

Beatles.
Used to listen to My Dad's Sgt Pepper's album as a wee nipper... and never quite cottoned on to the correct lyrics. Used to sing it like this....

Lovely Rita, Meter-maid, "notingham-castle, Shropshire"

No... I have no idea what I was thinking. doesn't even sound remotely like "nothing can come between us"

The geographical balls-up alone makes it cringe-worthy.

Plus.. Mick Hucknell's "bunny's too tight to mention" fantastically re-produced on rathergood.com
( Humpty Dumpty was Pushed Is unaware of having any limits., Thu 27 Jan 2005, 13:36, Reply)

Primark Scream
Get your socks off
Get your socks off, honey
Shake them now now
Get’em off downtown
Get your socks off
Get you socks off, honey
(Chillumm, Thu 27 Jan 2005, 13:35, Reply)

Eurythmics
Even though I know the truth, I still hear the lyrics as;

"It's all right, babies come in bags", instead of "coming back".

Which is odd.
( Linbox has a serious dunk habit, Thu 27 Jan 2005, 13:35, Reply)

[witty title]
[something stolen from kissthisguy.com]
( munkt0n, Thu 27 Jan 2005, 13:35, Reply)

Just a misheard lyrics
(I live in Austria, just so you know)

My friend and me, we were listening to music in the car. Suddenly the radio channel decides to play "Who let the dogs out." My friend, who can't speak english, seems to like the melody and starts singing along "oo laka buk sah".

Hilarity
(GiveMeMyLiverBack, Thu 27 Jan 2005, 13:33, Reply)

to the tune of "You to me are everything"
"Poo and wee are everything
the sweetest pong that I can ping
oh, gravy....oh, gravy."

I'm almost ashamed to write the words down...
( thorpe www.voodootrombonequartet.com, Thu 27 Jan 2005, 13:32, Reply)

strangers in the wife
exchanging fluids


her name is Rio and
she wanks with her left hand

also, 'i walking in the air'
becomes 'i'm wanking in your hair'

also there's a megadeath song, which idon't know the real words to, cos i always sing
'hello me, meet the real me, and my amazing bearded wife'
( mictoboy shitting in your cunt since, Thu 27 Jan 2005, 13:32, Reply)

Not a song, but the wonderful rhymes of Roald Dahl
Hey diddle diddle, we're all on the fiddle
and we never get up till noon.
We only take cash, which we carefully stash
and we work by the light of the moon.

or

Marry Marry quite contrary, how does your garden grow?
I live in a flat with a 5 year old brat, so how in the world should I know?
(Squash, Thu 27 Jan 2005, 13:29, Reply)

Love is in your hair.....
...and it's dribbling down your cheek....
( B00gle-H00ps Looks like a horse, in a man costume..., Thu 27 Jan 2005, 13:24, Reply)

An Ode to Blink 182
To the tune of 'Aliens Exist' by Blink 182.

Hey Mom, there's someone in the back room
I hope it's not my crazy Uncle George
He always used to touch me, undress me and carress me, social services think i'm telling lies.

If only people knew that the abuse was real.
I'd show them all my bruises and my scars.
I got an infection, because of the erection, that he put inside of me that night.

Up all night long
he made kept prodding me with his schlong
And I know it must be late
He's been prodding me since half past eight
He humps me I don't know why
he came right in my eye.

(at this point we repeat verse 2)

Up all night long, he makes me oil him to his thong.
And i know it must be late
I've been rubbing him since half past 8 (we go a bit unimaginative at this point)
He forced me to do 2 guys.
They split me not like YOU.

Dark and scary
He's so hairy
Often brutal
With his dildo
(we couldn't think of anything here)

Repeat either chorus.

Yeah, so it could use a little refinement. But we made it up when we were 13/14.
(Deiwos - Woo I found my old name!, Thu 27 Jan 2005, 13:24, Reply)

Hymn
Mine thighs have seen the glory of the coming of the Lord.
(Robertvaliant, Thu 27 Jan 2005, 13:20, Reply)

Tragedy!
You got your hole and now you're up the pole,
Tragedy!
The condom split and you came through it" etc etc
( pxyzyzygy, Thu 27 Jan 2005, 13:20, Reply)

Poor Blinded Bears
Not quite the wrong words, more the wrong end of the stick, but hey...
My Grandfather (sadly now deceased) when he was a wee nipper, was quite puzzled by one of the hymns sung at his local Church. "who is Gladly the cross-eyed bear?, and what's he got to do with the church" he wondered...

[edit]
argh... someone beat me to it. *sigh*
(tomptytom, Thu 27 Jan 2005, 13:18, Reply)

3
I'll never forget
the smell of the sweat from under his forskin
magic moments when two hearts our careing
magic moments while my arse is tearing.

some manchester indie song now goes:
Let me put you in the picture
Let me show you what I mean
This lassagne is my dinner
And its made by lean cuisine

and an over heard one to the tune of three lions on a shirt:
3 cocks up my arse
one of them is bleeding

I keep singing the last one at work:-( I cant help it it just leaps out
( Vultan the real one I threw my little yellow shoe, Thu 27 Jan 2005, 13:16, Reply)

God Rest Ye Merry Gentlemen
In primary school, we would anarchically sing "God rest ye geri mentalmen", employing a cunning spoonerism to imply that the men in question are a) mad, and b) geriactric.

Quite ingenious for a bunch of 9 year olds, I thought.
(scoob666 Ain't Nuthin' Ta F' Wit, Thu 27 Jan 2005, 13:16, Reply)