The Lite Bulb

The limerick packs laughs anatomical
Into space that is quite economical.
  But the good ones I've seen
  So seldom are clean
And the clean ones so seldom are comical.

Unknown Origin

This limerick goes in reverse
  Unless I'm remiss
  The neat thing is this:
If you start from the bottom-most verse
This limerick's not any worse.

Web Cartoonist Zach Weiner, 

author of SMBC-Comics, 

wrote a reversed limerick that makes sense read top-to-bottom, 

and vice versa.

anti~limericks

There was a Song man of A Pan
Whose limericks never would scan.
  When asked why this was,
  He replied "It's because
I always try to fit as many syllables into the last line as ever I possibly can."

Unknown Origin