On several occasions I've had a WP Network silently fail on me.
The blog's tables are not created and there are no error messages anywhere apart from all the message regarding the missing tables.
On one occasion it seems it was because I was logged into the main site and several sub-sites at the same time and recently it seems it was because of the Defensio plugin which was network activated.
But those are just assumptions based on a lengthy trial and error process. It's almost impossible to debug with the current tools.
So, it would be great if an exception could be raised when something like this happens. Something that could then be written to the PHP or WP log.