So since installing Ubuntu 12.10 Quantal, I've been plagued by strange authentication errors. I thought for some time that it was related to my use of LDAP for domain admin authentication, but it turns out it's more devious.
I run Gnome3. That means when I launch something like Synaptic I typically pop the menu up, type in "syna" and hit enter to launch the app.
The first thing Policykit does is open the password dialog.
I would then type in my password (I'm a fast typist) and hit enter. At this point it rejects my password even when correct.
Turns out, the enter key to launch synaptic is ALSO submitting the empty password dialog most of the time. It's not waiting for a keyup event before interpreting the keydown event on the password dialog, so simply holding the enter key at launch of whatever triggers the password dialog to submit a blank password.
This also occurs when using pkexec from the command line. I've filed a bug report.