So, there was a recent experiment that suggested that Neutrinos might travel faster than the speed of light.
This has massive problems for how we understand lots of really basic physics. In particular, particles that travel faster than the speed of light may violate causality (the order in which events happen, the repercussions could precede the cause!), and travel backwards in time.
So the joke is that the joke is back to front, the bar tender already talks to the neutrino before it has arrived!