Move this to the Glitch Discussion thread if this is the wrong subforum. (It relates to game mechanics more so than glitches and can be legally done via doubles)
Basically, when a Protean mon uses Curse, it targets the user, changes the user's type to ghost and then the Damage Over Time effect of a GhostUser!Curse is applied on the user.
I wonder how Protean + Roost would work. My guess is that it'll change the user's type to Flying, and then remove the Flying type and become Normal type. I have this reasoning because from Gen V onwards, a pure Flying type using Roost becomes Normal type (unlike ??? in Gen IV).
Thank Sleepy Jirachi for this find.