GoF Sphinx's riddle: Did Harry get it wrong?

[Amanda Coleman]

No: HPFGUIDX 130410

tigerpatronus at yahoo.com wrote:
"Next tell me what's always the last thing to mend,
The middle of middle and end of the end?"

The last word on the first line: "mend"


Amanda:

I think looking at this that this couplet is refering to the letter 
'd'.

Last thing to mend? ==d. (if looked at literally)
Middle of middle? =Ý (unless we're pedantic and say it's nothing
which I don't think JKR would do to us, at least not in conjunction
with the other two clues ie 'last thing to mend' and 'end of the end')
End of the end ==d.

Now.. singly I wouldn't put much credence in the 'd' being
significant, but they all reinforce one another and that seems a
Rowlingesque Riddle to me.

Though... having reasoned out that the couplet is referring to the
letter dee I am stuck for ideas of what significance this holds.

Amanda