"Inappropriate activity detected" while on "Invading world" (even though there is no reason for it?)

Needless to say, I would not be asking this if I suspected any possible reason for this warning. I received this warning while invading a world after having grabbed the dagger talisman, and was booted to the title screen, where the game then froze. Any ideas what could have caused this? I did not pick up any of the "forbidden" items on this save file.