Mystery Molecule!

This is a detective story. If you know the answer, please don't reveal it (though I'd be pleased that you announce that you know it). (Anyone remember when Psycho came out? Hitchcock made the audience promise not to tell).
Many years ago I used to synthesize compounds. Not very well, and not very safely. That's another story. My main interest was structural chemistry – specifically small molecule crystallography (proteins were rare then). I did this reaction (I am not telling you what went in, but it was in the area of coordination chemistry with organic ligands). One particular reaction didn't give what I expected but instead a very few, rather pretty, rather small (< 0.5 mm) dark orange crystals appeared. Too few for a chemical analysis (you had to burn them then) – no mass spec – not enough for an NMR or even an IR. But crystalline. So I thought I would find out by XRay crystallography.
So I put it on the XRay camera and measured the cell dimensions. It had a molecular mass of about 250. I was excited. A rather unusual spacegroup (I shall withhold this from you, readers, like a good author). And then suddenly I realised what it was.
I expect that by now all experienced chemical crystallographers will know what the material was. It is probably the most inadvertently studied molecule in the world. I have seen several re-determinations of this crystal structure. In crystallographic repositories like the one we are building in the SPECTRa project. And now, when it is common to mount-and-shoot (since a crystal structure can be determined in an hour or two) I suspect it happens once a week somewhere in the world.
So by know you either know what I am talking about or, I suspect, are completely bemused. If you are in the latter category I might release some more clues later. But I'd like to be confirmed in my suspicion that crystallographers will know the answer.