- depending upon the file system.
For instance, if you used ext3 then mkfs.ext3 is going to put backup super blocks all over your disk. If you then setup a hidden volume later on, some of those backup super blocks are going to get over written. An attacker - to whom you've been forced to reveal your outer volume password - could easily discover that the backup super blocks aren't the same as the real super block and deduce that you're using a hidden volume that you didn't tell them about. You could, when formating, tell mkfs.ext3 not to use any backup super blocks - but that also might look a bit suspicious. Just food for thought.
A computer scientist is someone who fixes things that aren't broken.