hi,
quite new with blender, don’t understand this (simple?) thing :
a cube, two lamps (one spot), nothing else … and a shadow line on the upper face of the cube.
Where does it comes from ?
file cube.blend is here :
http://perso.wanadoo.fr/musee.dinosaures/blendtmp/cube.blend
make clipstart value smaller (about 5)
is that what that little grey line is from. i have gottan this on a few of my models and was very puzzles
merci beaucoup ! 
it works, but i still don’t understand why… if clipstart is higher than the distance between the spot and the object, there should be no shadow at all?
No, the clipstart and clipend values must contain the objects to be illuminated. The clipstart value must be lower than the distance from the spot to the nearest illuminated object. Make sure the ilumination distance is high enough too.
One thing that confuses me is why the clipend value can exceed the illumination distance value. Maybe just an implementation thing.
The difference between clipStart and clipEnd controls the resolution of the buffer used for shadow calculations. Minimizing the distance makes for more accurate shadows.
As for exceeding the illum. distance, we just give you the knobs. You can twiddle them however you like.