The mathematician finished calculating the equation, the equation to which he had devoted the last several years of his life. With it, he would be able to determine the exact motion of every single subatomic particle in the entire history of the universe, Heisenberg be damned. Specifically, the equation used the present location of particles and atoms to calculate their past and future locations and movements.
First, he had to test it, and since he couldn't test the future he would have to do so by seeing if the past locations of atoms that the equation “predicted” matched the historical record. After a few hours, he was able to determine that it predicted past events with complete accuracy. Both major and minor events lined up perfectly with what history recorded. Unfortunately, the historical record was less specific and woefully incomplete -- for now -- but the events the equation calculated lined up perfectly.
As the magnitude of his discovery sank in, he began to wonder whether he could use the equation to determine the thoughts people had had in the past. Thoughts, after all, were just the movements of atoms in the cortex. Why should the movements that take place within our skulls be radically different from those that take place outside them?
(Of course, this raises the question of how he knew the physical event “Molecule A interacted with Molecule B in Fashion C” translates into the thought “My soup is cold”. This is a story in and of itself. Unfortunately, it's a long and boring story, so we'll skip it.)
He tried it out, and to his delight, received amazing results. He determined what Julius Caesar’s last thought was (the Latin equivalent of “oh crap”), what young Marco Polo’s thoughts were when he was first brought before the throne of Kublai Khan (strangely the same), and what Mozart’s thoughts were when he was composing his 40th symphony in G minor (which wasn’t as interesting as just listening to it). He determined the exact time and date that Copernicus first seriously considered the heliocentric system over the geocentric one.
This last step got the mathematician thinking. Calculating the equation made him one of the most important scientists who ever lived. What would he find if he used the equation to determine what his own thoughts were when he finished calculating the equation? Since this was the very recent past, it didn’t take that long to figure out. But the answer baffled him.
According to the equation, he calculated the equation wrong.