I already had a couple of akonadictl fsck runs, which resulted in a bunch of of missing external parts to be found (mostly on items in (disconnected) imap resources I think, not sure if solely in such items though).
I have no idea how that can happen and the first guess of a faulty filesystem seems unlikely that I'm having these problems since months and the system is otherwise running just fine. I had to shutdown the computer a couple of time when sleep failed or so, so that could be one reason.

I'm not sure if we can avoid the problem, but we have to be able to deal with it at least. akonadictl fsck can already detect the missing files, and clear the payload, so it's downloaded again once the item is requested (we might want to do that right away as part of the fix).
At least with some views missing parts can block a whole collection from being viewed, e.g. in akonadiconsole where selecting a folder always results in an itemfetch of the fullpayload 1:*, meaning the job will fail even if only one single item has a missing external part and it cannot be downloaded atm.

So there might be several different approaches, such as doing partial fetches in ItemFetchJob, detecting the problem in the background and fixing it before somebody notices (e.g. running fsck on startup), or avoid the problem altogether if thats possible at all.

Reproducible: Sometimes