Typesetting the biggest prime
Responding to a query from an NPR science correspondant about prime numbers, Hoefler & Frere-Jones researched the lengths involved when typesetting the largest known prime number, which has almost 13 million digits.
Joe liked the idea of measuring how long this number would be if it were set in type, which immediately called into question the choice of font. The number's length would depend chiefly on the width of the font selected, and even listener-friendly choices like Times Roman and Helvetica would produce dramatically different outcomes. Small eccentricities in the design of a particular number, such as Times Roman's inexplicably scrawny figure one, would have huge consequences when multiplied out to this length. But even this isn't the hairy part. Where things get difficult, as always, is in the kerning.
In some cases, properly kerning the number resulted in a difference of more than 1000 feet for 12 pt. text.