I finally bought myself one of the new Enhanced Dwarven Crossbows (added by the Dawnguard DLC).
Boom - the Courier was on me right away:
"Calcelmo wants your Crossbow."
Not the first time I've seen this - I just haven't seen it in a long while.

I promptly forgot about him and smithed-up / double-enchanted the bow, just to put it on display in my Hearthfire armory.
(it's kinda heavy to actually carry around)
Anyway it wouldn't let me rename it. I knew that was because of Calcelmo.

I hadn't seen him in ages so I actually bought another crossbow and took it to him.
He's all excited and offers double what it's worth.
That's how Calcelmo rolls, so we do the deal and I'm outta there (can't stand Markarth).

Then I notice that Calcelmo never took the crossbow...
(which used to be an issue, but had since been fixed for a long time now.)
Instead - the devil - he took my Black Book: Untold Legends
Ahhhh, bloody Skyrim... STILL finding new ways to freak me out, after so many hours played!