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Hubble once observed... wait for it... wait for it... THE SUN.
Oh, I got you with that one, didn't I? Admit it: you had no idea that Hubble actually and for real once observed the Sun, on purpose. I didn't know about it for a long time, until my friend and fellow astronomer Glenn Schneider clued me in. Glenn is a surprising guy in many ways -- he chases solar eclipses all over the planet, for example -- but this one was a doozy.
He has the whole story on his website. The short version is that some kinds of electronic detectors get extra electrons trapped in them, kinda like plaque in your arteries. One way to flush out these extra electrons is to flood the detector with ultraviolet light. The chips used in the original Wide Field/Planetary Camera launched with Hubble suffered from this, so they needed that UV flood. And it turns out there's a fairly bright source of UV light in space...
Maybe you see where this is going.
So the engineers rigged WFPC with a little mirror that stuck outside the camera. This part of the camera was actually mounted flush against Hubble's side, so the mirror stuck out from the 'scope like a wee periscope (there's a picture on Glenn's site that'll help). It faced backwards, towards Hubble's aft end. The great observatory was then pointed in the opposite direction of the Sun so the rear-view mirror was facing the Sun, and the sunlight was channeled right into WFPC.
The result is the image above: a bona-fide 100% actual image (well, mosaic) of the Sun taken by the Hubble Space Telescope.
How freaking cool is that?
