You should ask someone else about this, but just consider the red supergiant Betelgeuse. Betelgeuse is about 500 light-years away. For all we know, Betelgeuse may "already" have gone supernova. That is, if you were able to magically transport yourself to a place in spacetime within a few light-years of where Betelgeuse ought to be, you might find that the star is gone. (Of course, if it had already exploded as a supernova,
a supernova remnant would remain.)
As far as we know, a star that is about to explode as a supernova makes no outward signs of the upcoming event. It just suddenly explodes. That's why you can't say that a particular star is obviously going to explode next year. If that was the case you could carefully monitor the star and just wait for the tremendous fireworks. But it doesn't work like that, and Betelgeuse, for example, might go tomorrow or in a million years.
What do we mean if we say that Betelgeuse might go supernova tomorrow? We probably just mean that we will see the fantastic supernova in the sky tomorrow. But if we see the supernova tomorrow, then the explosion really happened 500 years ago. The star was obliterated 500 years ago, but the news about the star's demise - the brilliant light of the explosion - will reach us only 500 years later, after traveling for 500 years in our direction at the speed of light.
Keeping Betelgeuse under extremely close scrutiny to catch the supernova early is no use, since the actual explosion may not happen yet in a million years. That's because a star that goes supernova just pops, without any previous warning.
When it comes to the cloud which is about to plunge into the black hole at the center of our galaxy, the event has of course already happened some 26,000 years ago. But the "information about the event" - that is, any light flashes, radio waves, X-rays or the like that resulted from the dust cloud's death plunge - has not reached us yet. But we can see that we are going to have that information soon. We can't predict a supernova, but we
can predict the demise of the dust cloud near the black hole. We can see, judging by the information that has already reached us - although it got here "26,000 years too late" - that a cloud is going to fall into the central black hole. We can say, with confidence, that if we keep monitoring the black hole, then information of any fireworks that resulted from that event that happened 26,000 years ago will undoubtedly reach us in the pretty near future.
Ann