Old Mother Hubbard went to the cupboard,
To give the poor dog a bone:
When she came there, the cupboard was bare,
And so the poor dog had none.
It's always bothered me that "bone" doesn't rhyme with "none", especially since the other verses in the poem seem to try harder to rhyme the 2nd and 4th lines.
The third verse is worse:
She went to the undertaker's
To buy him a coffin;
When she came back
The dog was laughing.
Is this a case of a shift in pronunciation? Or does it simply not rhyme? (Or, does it rhyme, but only in certain accents?)