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Do you shoot raw or jpg?
The novice would answer that is is some kind of file corruption, but IMO probably not.
The thing is, most of the time these files are protected with their own checksums or CRCs. So if bits get corrupted during or after the file is written, it will, in very high likelihood, cause the CRC to fail. And that will be recognized by a viewer and it will either reject that file or warn you (if the viewer is any good).
For instance, photoshop will recognize this. You'll get a "CRC error" dialog. If you don't get this when loading the file, it's more likely it was some kind of wierd camera problem that happened like the communication between the camera's sensor and the microcontroller, or the image was somehow corrupted after that. But, almost certainly, before it was written to JPG / CRW / CR2. Because once it's packed into that format, the CRC is computed, so every point in time thereafter, a (non-intentional) corruption would be recognizeable by a failed CRC.