Math-mongers among you will recognize the allusion to French mathematician Pierre Fermat, who wrote in one of his notebooks that there was "unfortunately not room enough in the margin to write down the 'truly wonderful proof' he had worked out for that theorem. He then died before writing it down anywhere else and generations of mathematicians have been going crazy trying to figure out what it might have been.
In number theory, Fermat's Last Theorem (sometimes called Fermat's conjecture, states that no three positive integers a, b, and c satisfy the equation an + bn = cn for any integer value of n greater than 2. Fermat's Last Theorem resisted proof, leading to doubt that Fermat ever had a correct proof. Consequently the proposition became known as a conjecture rather than a theorem. After 358 years of effort by mathematicians, the first successful proof was released in 1994 by Andrew Wiles and formally published in 1995. It was described as a "stunning advance" by a bunch of really smart people.
As I say, I haven't seen too much quality scribbling of that ilk on walls. "Call and response' ones are the best; you know, where party A writes one thing and then party B comes along and has his say, etc. I recall one, memorable only because of the follow-up, that said, "I like grils" Below that, another person had written "Idiot! It's g-i-r-l-s!" A third person had then written "Yes, but what about us grils?!"
The best bit of repartee I have read around here was one that started with a long quotation from the Bible, written in a neat hand, obviously by a serious-minded chap with lots of time on his hands. At the end of the quotation was the optimistic assurance that "the Lord will forgive all your sins." A few days later, some skeptical humanist had scrawled in the comment, "All sins except writing on lavatory walls!" A few days after that, yet a third person, whose philosophical stance is not quite clear from his comment, wrote, "Surely, you don't consider writing on walls a sin!" By now, I was making sure to get my six full glasses of water every day, and then some, just so I would have an excuse to go back and check up on the discussion. Sure enough, a final comment appeared shortly thereafter, perhaps by part B, but possibly by an unknown sympathizer. A few days later, some skeptic scrawled: "Well, ok," it said, "writing on walls may not be as much fun as adultery, but..." I suspect that the dot-dot-dot indicates that the gentleman was interrupted and that possibly he intended to come back and finish it after he had some more water. We'll never know, because some kill-joys in the post-engineers painted over it all a few days later. You know, I have mulling that line over, the "dot-dot-dot"-part and I think I have found a truly wonderful proof of how the phantom humanist was going to finish it, but unfortunately...