I use an external drive to backup my UMBP with Time Machine. This has been happening for quite sometime but I'm surprised that people can rely on Time Machine given how brittle it is. Nearly always if you have to turn off your machine by holding down the power button instead of unmounting the Time Machine volume even when it's not actively backing up you will end up with a corrupted HFS Volume. That typically can only be fixed by running fsck_hfs -f or Disk Warrior I personally feel that it's unacceptable to end up with HFS Corruption just because I didn't umount the volume when it wasn't in use. Secondly, this HFS Corruption leads to the volume not be mountable upon boot. I'm really having a hard time putting my trust in Time Machine.