It should not be possible to edit the Anonymous user details via the 'Manage Users' control

You allow the user "Anonymous" to be edited via the Manage users screen. The support issues I typically saw were people who pulled up that user record via the 'Select anonymous user' checkbox and proceeded to edit the user details only to find out that they should not have done that. They find out the hard way that if they enter an email address or password for that account (because they didn't understand what they were doing) it cannot be undone except through a db edit again bypassing the ACP UI. Those fields should not be editable, the Anonymous user record should display in read only mode as it is merely a pre-defined system account much like the pre-defined groups which cannot be edited or removed. It also should not be able to be removed from the 'Guests' group or added to any other group. Fixing these design flaws would improve the usability by preventing users from creating an error state in the first place. "An ounce of prevention is worth a pound of cure."

A_Jelly_Doughnut: What bad behavior comes of this? I do not notice anything obviously wrong coming from assigning an email address and password to anonymous. Sure, you can't un-do it, but that doesn't really mean anything is broken.

Editing Anonymous is important for certain tasks, like changing the style that guest users see.

What bad behavior comes of this?

You answered your own question....

Sure, you can't un-do it ...
but that doesn't really mean anything is broken.

That is most definitely "broken", the following support topic is a perfect illustration of why it should not be possible in the first place.
http://www.phpbb.com/community/viewtopic.php?p=10037975#p10037975

Editing Anonymous is important for certain tasks, like changing the style that guest users see.

You really don't want to get me started on that thorn in everybody's side do you? The absurdity of having to actually set "My board style" under user Preferences for the user "Anonymous" rather than having it automatically pick up the board default setting like it does for Timezone and Language is laughable. Do you have any idea how many support topics that completely illogical inconsistency creates? Guests(aka Anonymous) should not have "My anything" as they are not unique individuals but instead every single un-logged in board visitor.