  In the famous thought experiment known as Wigner's friend, Wigner assigns an
entangled state to the composite quantum system consisting of his friend and
her observed system. In the context of this thought experiment, Brukner
recently derived a no-go theorem for observer-independent facts, i.e. those
facts that would be common to both Wigner and his friend, and initial
experimental tests have given it strong support. In this article, we
demonstrate a loophole in the theorem that manifests in the presence of a
horizon even if Wigner and his friend are on the same side of the horizon. The
loophole requires that Wigner and his friend both unambiguously demonstrate
that they are in inertial frames. We then argue that unambiguously proving that
they are both in inertial frames requires unambiguously proving the existence
of Unruh radiation. However, we also argue that if they cannot prove this, then
there are still limitations on observer-independent facts that require that
some of those facts remain unknowable. As such, observer-independent facts
remain illusory.
