  It is commonly believed that the ringdown signal from a binary coalescence
provides a conclusive proof for the formation of an event horizon after the
merger. This expectation is based on the assumption that the ringdown waveform
at intermediate times is dominated by the quasinormal modes of the final
object. We point out that this assumption should be taken with great care, and
that very compact objects with a light ring will display a similar ringdown
stage, even when their quasinormal-mode spectrum is completely different from
that of a black hole. In other words, universal ringdown waveforms indicate the
presence of light rings, rather than of horizons. Only precision observations
of the late-time ringdown signal, where the differences in the quasinormal-mode
spectrum eventually show up, can be used to rule out exotic alternatives to
black holes and to test quantum effects at the horizon scale.
