The Thirteenth Floor is one of my favourite movies of all time. Truly mind-mangling, it is what The Matrix wishes it could be but isn't.
In Derpy's case, where is the top of the helter-skelter? At what level is she and where is the real world... if there is even such a thing? Is she a person trapped in a simulation... or is she a simulated person trapped in a simulation created by the simulations in the simulation?
It's questions like that which make The Thirteenth Floor not the sort of film you can watch without having drunk coffee beforehand
It's about a guy working on a Virtual Reality project - a complete virtual world with artificially-intelligent simulated people. He finds that the AIs are so sophisticated that they could hypothetically get out by 'brain-jacking' the team's bodies. However, as he studies the AIs, he finds he can't just turn them off - they're fully self-aware and sentient.
Things start getting weirder. He starts finding all the tell-tales of being in a simulation in the 'real world'. He starts wondering if he has been trapped in the simulation by some AI that has stolen his body... Then things get even weirder; I won't spoil your fun by going any further!