I really like the story of the astronomers who have measured details of the supernova explosion which Tycho Brahe observed in 1572. They didn't travel back in time to look over his shoulder, though. They just used light from the very same event that got scattered by some cloud. With an object so ridiculously far away (though still in our galaxy), a slight deviation in the light path can be enough to delay the arrival by 436 years so we can see it.
The conclusion is rather boring (the supernova is now officially classified as a type Ia, which is what people suspected it to be anyway), but the idea that you can see light from the same flash observed by a Renaissance astronomer, that is just brilliant.
This is published in the current issue of Nature, page 617, with a very readable News and Views piece on page 587.