I've never once seen a TB locked.
But I have. Dozens and dozwns of times. It happens all the time when someone abuses the geocaching.com system by deliberately allowing people to log a Trackable virtually, meaning they pass out the tracking number to people who have never seen the trackable. Believe me, you really don't want to get a TB tattoo and then get it locked down. You might hate yourself for it.
Thanks Eartha, I wasn't aware of that. My offer is officially rescinded, you'll have to come find me on the trails or at an Event