Charlie's middle name is Edward.
This is the second time that Don has had his gun stolen, the first time is in the second episode of the first season, Uncertainty Principle (2005). As he says this one is his second and he's had it six years, this gun is likely the one that replaced that one.
Don has been an FBI agent for 15 years, and worked with David for six years.
Last show of the series.
This episode has a callback to the first episode of the series, Pilot (2005). In each, Don borrows a shirt from Alan.