In some planes the head position keys (PGUP, PGDN, UP, DOWN. LEFT, RIGHT) don't work correctly.
Example: I press keypad 5 + 4 for left upward view then RIGHT. Now I want go back to LEFT but the LEFT key doesn't work. I didn't test this seriously so I can't say if there is a pattern. Only thing I can say is that all head position keys always work in the default forward view.