The Trauma Certification course that April does with the interns was what Owen made her go through in That's Me Trying (2010).
When one of the interns tries to cheat his way through the trauma certification by paging Kepner away, the interns reveal that Kepner's own cheating during her certification was well-known, and Deluca refers to it as "the ambulance story." This refers to episode 7.3 "That's Me Trying" in which Kepner, frustrated that her "patients" keep dying and the imaginary helicopter is hours overdue, insists on using a real ambulance parked nearby to "rescue" the patients.