During the shooting of the cab scene, Jason Segel's (Marshall) script didn't tell him the real news, but instead it said the Lilly was going to say she was pregnant. Therefore, Marshall's reaction is actually Jason Segel's very real reaction. The producers of the show did this because the show focuses on milestones of young adult life, and they wanted to make the shock and dismay of unexpected events such as that scene very real.
There is a countdown throughout the episode - from 50 to 1. The countdown is as follows:
Neil Patrick Harris is credited twice for this episode, once in the opening as a member of the main cast and once at the end as "Dr. John Stangel."
The glasses Barney is wearing in Dr. Stangel's office are The Sensory Deprivator 5000 that Ted invented in the season 2 episode Monday Night Football.
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