Have you tried simply restarting Aperture to see if it fixes the issue?
If that doesn't, you can probably select the image and select "reprocess master." Usually a full database repair shouldn't be necessary. As to why it happens, I don't know. I've seen this as well, but like twice ever and I've been using Aperture since it was released in 2005.
Where do I go to reprocess master? Couldn't find it anywhere.
In Aperture 3 this option is in the "Photos" menu:
Aperture's main menu bar > Photos > Reprocess Original
Does this always happen with the same card? When have you last reformatted the card? Perhaps the filesystem on the card needs repairing or you should try a different card.
I see this regularly ( one or two files, every other time I import?) with the built-in SDHC card reader on my rMBP. What I do is simply note which files are corrupt, immediately re-import those files, and then delete the corrupt ones (sorted by date, they are side-by-side with their non-corrupt twin). Iirc, only once have I had to re-re-import a file that failed to import properly on the first try.
I reformat (in camera) each card every time I fill it (and recommend this practice), but IME (casually observed and not tested) the problem originates with the built-in card reader, and not with the card. How are you importing?
Perhaps only a coincidence, but I use Sony cameras. I don't recall this ever happening with a CF card though. (The rMBP does not have a CF reader.)