UV Mapping behaves very weirdly I was trying to texture a car, but then I realized that some faces dont have their UV map applied.
I cant really explain what's happening. It looks like that there are two kinds of groups; when I select the entire model and apply a UV Map to it only one of the groups of faces actually uses it. The other stays with their current UV Map.

The car model: https://docs.google.com/file/d/0Bz5XxZPILhgEc3h2NVk4QnZPalU/edit
There is a mode in the UV Editor called Sync Selection.  When Sync Selection is enabled; selecting the entire model in the 3D Viewport will select everything in the UV Editor; 
however! when Sync Selection is disabled this will not happen.  

To select everything when Sync is disabled you must first select everything in the 3D Viewport and then again in the UV Editor.  This mode allows you to work on UV coords independently but will often cause the problem you are describing. 

Make sure that everything is selected in the UV Editor and then reapply the texture.
You might want to keep that little button activated all the time until you get used to it.