There's always vacuum decay. Simply put, the entire universe could be in an unstable "false vacuum" state and all it would take is something suitably cataclysmic, yet of a form beyond that we can detect, and what we know of as spacetime would unravel from that point outwards at light speed, undetectable until it hits us. If you've ever seen those slow motion videos of a water balloon popping, it'd be a bit like that, with us on the bit of balloon that hasn't quite "realised" that half the balloon is gone already.

But then, predictions range from "complete annihilation of all things" to "imperceptible shift in cosmological constants that doesn't affect much at all". Heck, it might have already happened once or twice if that's the case.

But if it hasn't... POP!