When I tried to register to report bugs & improvements, I created a username with a space:
I duly got a message with confirmation URL, so the name with the space was accepted, but I could not log in at all.
I then created a new registration with the same name but without the space, and this worked! I consider this a bug. Spaces in names should be legitimate characters.
Furthermore, I now seem to be "Robert Cailliau" anyway, with the e-mail I did give for the first registration (different from the second one) and can not login with the name without the space.