As Cassini weaves its way around the multiple moons of Saturn, it's not really a coincidence when one gets in the way of another. As a matter of fact, it's a guarantee. These are called mutual events, and when Cassini dove past Dione, it saw this terrific view of Mimas peeking out from behind it:
Nifty, huh? [Click to encronosenate.]
Dione is nearly 3 times larger than Mimas (1100 versus 400 km wide), but Mimas was also more than 6 times farther away, making Dione loom nearly 20 times larger in this shot. I like how you can't really see the unlit side of Dione, but Mimas marks it pretty well, sliced in half by the edge of the larger moon.
Funny, too: I was thinking to myself that if Cassini was in position to catch this shot, then it should have also caught Mimas when it was on the other side of Dione, the lit part. Well, seek and ye shall find: I searched the Cassini raw image archive and found it! I put a small version of it here; click to embiggen. You can just barely see a small segment of Saturn's rings in the lower left corner, too.
Neat! I like it when stuff makes sense. While this alignment is rare to see from Earth -- we're a lot farther away, and the geometry has to be precise -- we do see moons transiting across their parent planets, and, far less often moons in front of moons. But what's rare to us is common to Cassini, with its front row seat to this amazing system of worlds.