See http://youtu.be/LP0vw18vR2I
1. Place a NATO Rifleman at 021057
2. Try to walk up the stairs of the two-story building
3. Select rifle
3. Notice you cannot
4. Lower rifle
5. Notice you can go up now
6. Switch to rifle
7. Start running
8. Notice you cannot get up the stairs again
9. Switch to walk or tactical
10. Notice you can go up now