There was a young man from Lahore
Whose limericks stopped at line four.
When asked why this was,
He responded, “Because.”

Also:

There was a young man from Iran
Whose poetry just wouldn’t scan.
When they said, “But the thing
Doesn’t go with the swing,”
He replied, “Yes, I’m aware of that, but I like to put as many syllables in the last line as I can.”