I have 2 Transcend x133 16GB CF cards used on a 7D in Raw+Jpg, and they failed when I reach the 8GB capacity. No error messages, I can continue to shoot but only a few empty files are recorded. No rescue software (pc-smart-recovery, PhotoRescue...) can retrieve the lost files, PhotoRescue is able to recover the last incomplete file. If I try to fill the card with pictures from my PC, the same happens: only about the first half is visible in the 7D, even if I have no problem with the card in the PC. Formating the card with the PC (FAT32) instead than with the 7D doesn't change anything. My firmware is v1.10e. Looks like a problem of capacity limit for me.
Be careful, there is no warning, except that as you continue to shoot, the number of remaining frames doesn't decrease anymore...! I lost 2 days of pictures (one on each card before understanding what's going on!