problem with border selection

Beginner problem, probably.
With Blender 2.71, in edit mode, I can select vertices, edges or faces, but, now, as soon as I border select a part of my 3D view (ALTB), I can’t select anything anymore…
And, I think I did this (selecting in border select view) a hundred times without problem, since I work on this blend file. Is there a button that I might have toggled without noticing ?

Dont’ bother !
5 min later, it’s working again.
Too bad I can’t understand why…