Could that be due to a failing SD card or flash memory? The uncompressed data might be getting corrupted too, it just wouldn't return you an error depending on the file type (text documents for example would never error, they would just end up with garbled text).

Based on your other comments, if you're using an old 128mb SD card, this seems kind of likely. I've had many SD cards and USB sticks go bad after files have been copied on and off of them too many times.