Problem with ilumination

Hello there, i’m following the book “Blender Foundations, The essential guide for learning blender 2.6”, i’m currently on chapter 5 which deals with lightning, and i’m having this strange problem, i don’t know why the light keep going through the edge of the cube, i have tried everything, set the angle high so i covered the whole cube (just in case the problem was due to the light cone going through the walls or something). any help would be appreciated :).

http://img803.imageshack.us/img803/4907/problemp.png

This is due to your use of buffer shadows. Lower the bias to about 0.5. That should do it.

From the wiki:

Bias The Bias numeric field can have a value between 0.001 and 5.0. Bias is used to add a slight offset distance between an object and the shadows cast by it. This is sometimes required because of inaccuracies in the calculation which determines weather an area of an object is in shadow or not. Making the Bias value smaller results in the distance between the object and its shadow being smaller. If the Bias value is too small, an object can get artifacts, which can appear as lines and interference patterns on objects. This problem is usually called “self shadowing”, and can usually be fixed by increasing the Bias value, which exists for that purpose! Other methods for correcting self shadowing include increasing the size of the ShadowBufferSize or using a different buffer shadow calculation method such as Classic-Halfway or Irregular.
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http://wiki.blender.org/index.php/Doc:2.4/Manual/Lighting/Lamps/Spot/Buffered_Shadows

sorry, but for strange reason i wasn’t notify about this reply, any way, its not the bias, actually setting the bias less than 1 is giving me less light going trough the walls, but i’m getting some circles on my light, and when bias is 0.001 i still have some thin light going trough the wall. i was thinking the problem was the cube, but replacing ti gave me the same problem, ¿ is this some kind of bug ?