Today, Moustakas and I pair-coded a world coordinate system (WCS) determination system in IDL which not just aligns images with the USNO-B1.0 astrometric standards catalog but also with one another where there are overlaps. The algorithms are naive and need some work, but the system works and produces good quality, consistent WCS. We also pair-coded a simple image combination and/or mosaicing system, because swarp (the favorite of the in crowd) doesn't understand SIP headers (the current standard for polynomial-distorted tangent-plane projections of the sphere onto the plane).
This was a great day: All coding, no committees, no grant proposals, no meetings, no email! Also, pair coding is substantially better than just plain coding for a number of reasons, not limited to: (1) far fewer stupid bugs because you have a second pair of eyes, (2) fast development because each person eggs the other on, (3) no API/interface issues because all code is written by both developers simultaneously (ie, it is far faster than having each developer write half the code and hoping the two halves
talk), and (4) it is more fun.