In honor of St. Patrick’s Day, I’d like to talk about the limerick, a snowclone which has a whole database devoted to its variants. Here is a good approximation of the de-specified form of the limerick:
There once was an X from place B,
That satisfied predicate P,
He or she did thing A,
In an adjective way,
Resulting in circumstance C.
According to Wikipedia, there is no strict requirement that a limerick adhere to this form; that is, while “Hickory Dickory Dock”1 does not fill the above template, it is still technically a limerick. This kind of variation illustrates how the limerick as such isn’t a snowclone, any more than a haiku is a snowclone–restrictions on the number of lines and the rhythm or rhyme of a poem do not a snowclone make. That this template could be devised in the first place is what satisfies my requirements for snowclone-hood.
Edward Lear popularized this form in his 1845 Book of Nonsense, with such entries as:
There was an Old Derry down Derry,
who loved to see little folks merry;
So he made them a book,
and with laughter they shook
at the fun of that Derry down Derry.
Limerick-writing seems to be quite the alive-and-well pasttime, if this database is any indication. Here are some more from “the top 150” there:
There once was a man named Bertold
Who drank beer when the weather grew cold
As he reached for his cup…
“NEEEEVER GONNA GIVE YOU UP!!!”
Oh, snap! You just got limerickrolled!
I’ve now been disabused of the notion that the modern limerick has a bawdiness requirement, but I like how this one is suggestive:
There once was a maid from Madras
Who had a magnificent ass.
Not rounded and pink,
as you’d possibly think;
It was gray, had long ears, and ate grass.
1
Hickere, Dickere Dock,
A Mouse ran up the Clock,The Clock Struck One,
The Mouse fell down,And Hickere Dickere Dock.
The Snowclones Database 
Lear’s limericks are unusual by modern standards in that the first line is at least partly repeated in the last — there is no turn, or twist, just a wrap-around.
Two (or three) favorite frame-breakers:
There was an old man
from Peru, whose lim’ricks all
look’d like haiku — he
said with a laugh, “I
cut them in half, the pay is
much better for two.”
–Emmet O’Brien
There was a young fellow from Bees,
Who was stung in the neck by a wasp.
When asked “Does it hurt?”
He replied, “No, it doesn’t.
It would have been terrible if it had been a hornet.”
–W.S. Gilbert, slightly improved by oral tradition
With reference to the bawdiness requirement: Daniel Dennett quotes a delightful example which I will attempt to reproduce from memory.
There was a young lady named Tuck
Who had the most terrible luck.
She went out in a punt,
And fell over the front,
And was bit in the leg by a duck.
Oh man, so I found this site the day before I read this, and I had to share it:
http://limerickdb.com/?latest
It has limericks about everything from science:
Black Holes are fantastic creations
Whose physics defy explainations
Time and space they so bend
Such that few comprehend
The gravity of these situations!
to broadway:
Prohibition Chicago’s quite glossy
Its feminine killers are saucy
Every murderess there
wears just underwear
and the dancing’s all done by Bob Fosse
Other topics include the charge of the light brigade, Jesus, limericks, and talking ducks.
Look! a limerick about a snowclone!
There once was a meme that I knew
’twas on Digg, Reddit and Slashdot too.
So I know its quite old,
and its charms have grown cold,
but in Russia: Limericks rhyme you
Hans Alfredson insists angrily, in his 1976 book on limericks, Svea Hunds Limerickar, that one of the rhymes needs to be at least bisyllabic while the other one monosyllabic. He wouldn’t approve of the “predicate P” one.