In Rogue One, Jyn wants to find the plan for the Death Star among multiple plans in the Imperial citadel archives. She identifies which plan they need because the plan bears the codename Stardust, which is the same name her father Galen Erso used to call her. Jyn points this out explicitly in the film.
However, Galen Erso apparently didn't have access to the plans of the Death Star. How did he find out about that codename then, so that he can drop that hint to his daughter? Alternately, if Stardust was his daughter's nickname first, then why did the Death Star get that as a codename?
This question came up from the answer "Why did the rebels need the Death Star plans?", which claims that Galen didn't have access to those plans.