The error on your camera/computer you never wish to see!
Half way through a shoot the other day I had a dreaded card error on my camera.
Not to worry, I just changed cards and continued with the shoot knowing that although the card was probably corrupt, I still had the backup card in the camera and the images would be on that.
When I got home, the original card was in fact corrupt and unreadable, but that was fine as all the images from the day were still on the backup card as I expected. Nothing lost, yes ... nothing at all was lost!
This is what separates professional photographers from amateurs or friends who have a "good camera". I have numerous backup camera bodies, backup lenses and dual card slots in the camera bodies - so in the event of kit failure, I'm still covered no matter what. Imagine if this happened on your wedding day, that your "friend" was photographing - no backup kit or cards would mean no photographs from your special day, and its not as though you can re-enact the day!
Hire a professional like myself to do the job.