While phasing, a creature ignores difficult terrain and can move through obstacles and other creatures, but it must end its movement in an unoccupied space.
However, given a sufficiently thick wall that the player doesn't know about beforehand, if a phasing player tries to run through said incredibly thick wall and reaches the end of her movement... what do the rules say happens?
(Yes, there's precedent in earlier editions, but I haven't been able to find anything in DDI about this edge case.)