I do like a good mystery, so here is one courtesy of the Hubble Space Telescope:
In 2006, while looking for supernovae, the Hubble detected an optical transient unlike anything that has ever been seen before.
The object in the right-hand screen was first spotted in February 2006. It brightened steadily for 100 days and then dimmed at roughly the same rate, which makes it unlike any supernova ever seen. It also means it is unlikely to have been some kind of gravitational lensing event. There is no known object at that location in space. And spectral analysis has been frustratingly inconclusive (although there may be redshifted molecular carbon lines in the spectrum, in which case the object is about a billion light-years away).
For more details, see the HubbleSite press release.