When Charlie is speaking to Larry Fleinhart (roughly eight minutes into the episode) about his relationship with Marshall Penfield, he walks by a chalkboard with a number of equations written on it. Several of these can be clearly seen 8 minutes and 21 seconds into the episode. The lowermost equation, unlike the others, appears to have been included as a joke, as it reads: 1+1=2. Everything else on the chalkboard involves notational formalism related to more complicated mathematics, and this simple arithmetic equation is clearly unrelated.
Colin Hanks and Diane Farr previously starred in the tv show "Roswell." Diane Farr played the mother of Colin Hanks's character's friend.
A version of Charlie's speech about calendars and the inference that there are connected crimes they are currently missing was part of the auditions for the character of Charlie. David Krumholtz's audition tapes, including that version of this scene, can be seen on the season one DVDs.