I know this has been reported before, but I've noticed that I'm getting more and more Zone Transition Failures the longer I play the same character. They were rare at first, but now that I have a Level 10 character tromping around in Junkyard, I've started seeing them with greater frequency.
At the moment, I can't enter the 'Back Alley' area of Junkyard (preventing me from finishing Grover's optional quest), and I can't exit Junkyard back into the Hopper Caves (forcing me to use the Ferry to get back to South Gate Station). Not insurmountable, but mildly annoying.
EDIT - Upon returning to South Gate Station to visit the vendors, I've encountered more Zone Transition Failures. I can no longer visit Floors 6-8 (Medical, Engineering, and Agronomy).
EDIT x2 - Or the Hopper Caves.