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"The Broad Majestic Shannon"

"So I walked as day was dawning
Where small birds sang and leaves were falling
Where we once watched the row boats landing
By the broad majestic Shannon"

Was really...

"So I walked as day was dawning
Where small birds sang and leaves were falling
Where we once watched the ROBOTS landing
By the broad majestic Shannon"

This was posted by Miss Suzie Spitfyre at the FOS maillist.

"Dirty Old Town"

"I'm going to make a good sharp axe
Shining steel tempered in the fire
Will chop you down like an old dead tree
Dirty old town
Dirty old town"

Was actually...

"I'm going to make ME A FISHERMAN 
Shining steel tempered in the fire
Will chop you down like an old dead tree
Dirty old town
Dirty old town"

This was posted by Miss Suzie Spitfyre at the FOS maillist.

"Dark Streets of London"

"Now the winter comes down and I can't stand the chill
That comes to the streets around Christmas time"

in a way:

"Now the winter comes down and I can't stand the chill
That comes to the streets and I KISS MY ASS GOODBYE"

This was posted by Metukka at the FOS maillist.

"Boys of the County Hell" 
Misheard Lyrics:
Stay on the other side of the road 
'Cause you can never tell 
We're a bunch of good looking devils 
We're the boys of the county hell
Correct Lyrics:
Stay on the other side of the road 
'Cause you can never tell 
We've a thirst like a gang of devils 
We're the boys of the county hell

"Broad Majestic Shannon"
Misheard Lyrics:
We heard a siren from the ducks.
Correct Lyrics:
We heard a siren from the docks.

"Fairytale Of New York"
Misheard Lyrics:
The boys in the NYPD choir will sing in Colwyn Bay.
Correct Lyrics:
The boys of the NYPD choir were singing 'Galway Bay'.

"Fairytale Of New York"
Misheard Lyrics:
The boys of the empire singing high and singing low and gay
Correct Lyrics:
The boys of the nypd choir were singing 'Galway Bay'

"Fairytale Of New York"
Misheard Lyrics:
When the band finished playing, they held up the mall.
Correct Lyrics:
When the band finished playing, they howled out for more.

"The Broad Majestic Shannon" 
Misheard Lyrics:
Where we once watched the robots landing
On the broad, majestic Shannon.
Correct Lyrics:
Where we once watched the rowboats landing
On the broad, majestic Shannon

Source for the last ones: Am I Right - Misheard Lyrics.