Um, working fine for me today. :s
I have had intermittent DB errors when moving inventory stuff, sometimes it actually moved the item even though it had an error.
Other than that it can happen if:
* You've got a daft amount of items in a set.
* You've set a price on a set with over 50 items, sometimes completely screwing the set & items.
* There's a broken item involved, problem is you can't tell which one as the problem happens with everything.
Like I said, moving items in & out sets is working ATM, I moved a load around about 30 mins ago.