Noticed this on a friends new HL1 and then hurried home to see the same mystery on my XL-2.
Shooting, for instance, TV mode 25P. Engage exposure lock and set shutter speed to 50 and say my f stop is 8. Move camera skyward (or to a brighter scene) try to bring my f stop to 11 and the shutter closes down (black). Disengage exposure lock, re engage and the f stop is 11 or 16. In other words the camera will engage a smaller f stop automatically but won't allow me to manually go below f8. What gives?