Criminal Minds pays tribute to the BAU's early days -- and to the show's early days and one of the original profilers -- in season 10 episode 12, "Nelson's Sparrow," as flashbacks show the unit when it wasn't even the BAU yet. Back then, in 1978, Jason Gideon and David Rossi were just two of three agents in the BSU (the Behavioral Science Unit or "BS Unit" as others called it), and they were just building the foundation of their army.
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Going into the episode, I expected a few callbacks, I expected to hear Rossi and Gideon using terms for the first time that are used by the team all the time in the present. What I didn't expect is that this ends up being a very sad episode in quite a fitting way. It isn't often that the word "heartbreaking" applies to this show, but in this case, it does.