Bracketing would be the 1st and most obvious situation.
I did have an occurrence of extremely wild shutter shifts. Shutter speed went from 1/200 to 1/1000 or 1/8000 with no bracketing or other intervention. I was using a flash at the time and I tried removing the flash, resetting features and re-seating the lens to no avail. 2 button reset did the trick and it has not happened since.
Stuff happens - I once had a single JPEG show up in a batch of NEFs. The JPEG was part of a sequence of shots all taken within 30 secs. Never happened again.