Can not exit a mission without finishing it.
It's possible you were clicking a door (yes, even in a hillside) that was NOT the mission exit. You could have used your WW/Base/Pocket D porter to get out, though.
That would be my thought. I have NEVER seen a mission that I could not exit. Of course, FINDING the exit might be a challenge since there is no waypoint.
Paragon City Search And Rescue
The Mentor Project
Okay, this happened to me tonight.
I started the "Stop Wailers in spirit world" mission from Hardcase. Seeing as this is a clear-all on an outdoor map, you can understand how I ended up quickly filling up my enhancement inventory, and making a pretty big dent into my Recipe inventory, too. So I decided to leave, sell some things, and return.
Only I couldn't leave.
I got a "Can not enter" message at the exit. I was not allowed to leave the map. I can only assume that actually finishing the mission would work - I DCed before that happened.
Still, this was a nasty surprise, especially on such a large map with so many enemies on it.
-This Space Intentionally Left Blank.-