If you create a new user *without* a blog - just a username
And then create a blog for that user later
And then try and go to <sitename>/wp_admin/ (the Site Admin link in the meta block from the site home, or from anywhere other than your own blog really)
It fails to redirect to your own admin section; it just gets into an infinite loop and generates a browser error (in Firefox anyway, IE just loops)
This is not a problem if you create a new user with a blog, only if you create a new username only, no blog.
The obvious workaround is of course to prevent new users from making an account with no blog attached, but this is a pretty crappy workaround.
Using wordpress mu 1.2.4 and apache 2.0.58-r2