A variant on what has already been proposed here - the danger is again, from space, because astronomy and astrophysics is SO cool.
Blow the horns, sound the alarms, we are on a collision course... with a BLACK HOLE.
1. Noone would know about it. Normal, run-of-the-mill, friendly neighbourhood sized black holes (as opposed to wallmart sized monsters at the centers of the galaxies) are super difficult to spot if You are not actively trying to find them because You had clues that they are there in the first place. And it just so happens that this particular black hole is rather small and has been travelling in space without any companionship. No accretion disk, no large body to affect with gravitational field, nothing.
2. An astronomer spotted it because of luck. He was drunk and was bragging to his non-science buddies 'muh scienz' and typed random coordinates in the sky with his telescope at home and pushed 'track this spot in the sky'. He didn't see much of course at the time, but hangovered in the morning, he spotted something wierd with the image from the telescope, it was tad bit too... empty. Especially after the few hours of data gathering. So he found some free time on some real sized telescope... and wet his pants seeing the gravitational lensing. 73
It's like winning a lottery, spotting a black hole like that. Muh papers! Muh recognition! A few months later and 10 publications in PNAS, someone calculates (for the fun of it) the current velocity and direction of the black hole. Well, bad news, it's going to go straight through the solar system in ...
3. You can have this disaster happen with any time delay You want. The first effects may be seen in a month, may be seen in 100 years. Calibrate it however You want.
4. There IS no way to stop it. One does not simply change the trajectory of a black hole.
5. Put it close enough, and there IS no escape (it wouldn't be detectable without enormous amount of luck) until literally on our doorstep - and then it's too late to start building interstellar colony ships.
6. The sheer unprobability of this event is interesting. One type of religious people start arguing this is an argument for the existence of god, and that he IS mad. Other type of religious people praying day and night, hoping that it's just a test and in the end the reality devouring monster will be deflected by saviour. UFO theorists shout about Kardashev level LOTS civilisation which decided to eradicate humanity in this, rather baroque, manner. So what is the truth about this? Epic bad luck? Malevolent intelligence with a penchant for overkill of epic proportions? Angry god? Or just god testing the faithfull? Or maybe something more curious...? 337