When creating new diff through web interface,

Set the Visible to: to default.
Paste or upload a patch (used svn and git patches)
Do not add any repository name.

Exception is seen and diff creation is aborted.
Unhandled Exception ("PhabricatorApplicationTransactionNoEffectException")
Transactions have no effect: - This Revision already has that view policy.

Consistently reproduced in https://secure.phabricator.com and a local install.
We have not enabled Public view policy, so 'All Users' is default in our setup.

A low level debugging attempt:
OldViewPolicy and NewViewPolicy is same in filterTransactions() throwing NoEffectException.