I think I described it well enough!
But here is a picture of the shape, in one of the two Java programs (both mine) where I've seen this shape explode. I drew it a little bit inaccurately to make it clearer where the joints' hubs were. The purple shapes are static obstacles to keep the X from falling (there's gravity). The X explodes pretty fast, for many step-duration values. (That's the .023 down at the right.)
Incidentally, the 14 down at the right is a step-interaction count (the second argument to World.step()). Is 14 an unreasonably low number? I've found that if I crank that up to 40 or so, the explosion mostly goes away, and at 80 or so it seems to stop. But I had the impression that 10 was used in the JBox2D demos... regardless, comparably complex shapes made from distance joints are MUCH more stable than these exes I'm making.
In this thread, a dude links to a sandbox that's made in the D port apparently: viewtopic.php?f=4&t=2756&p=13783&hilit=sandbox#p13783
(search page for "blaze")
I tried drawing my X in that sandbox and it exploded even more easily than it did in my own sandbox. I cut off one leg, and it still exploded. I cut off two legs, and it didn't explode. (I saw all this behavior in my programs as well.)
Do you still want code? I'm telling you, it's as straightforward as it looks--the X explodes pretty reliably, whether it's roughly drawn or placed precisely in code. But I can whip up a small program that neatly demonstrates it if you need me to.
Surely I'm not the first person to draw an X....