A number of common misconfigurations for DAViCal can occur.
mod_dav conflicts with DAViCal
Symptoms: Stuff doesn't work, particularly iCal, though the admin interface is fine.
DAViCal needs to handle all DAV activity for the calendar paths so if you have to have mod_dav installed within the virtual host used for DAViCal you need to make sure that mod_dav is disabled for the subset of paths covering where DAViCal is installed.
iCal can't authenticate against DAViCal after OS X upgrade
Symptoms: Stuff doesn't work, iCal keeps asking for credentials, though the admin interface is fine. New DAViCal or OS X users in the same machine can connect to DAViCal just fine.
This is a problem with the OS X Keychain upgrade; the account is zombified. Delete all of the user's keys for the DAViCal server (they will be of kind = Internet password) using Applications/Utilities/Keychain Access, then open iCal and configure the user again. All calendars in the DAViCal server will populate iCal again. This may take a while depending on how many items are already in DAViCal.
Text after PHP closure in configuration file
Symptoms: Prompted to log in for each page in the admin interface.
If you have extra characters after the ?> at the end of your DAViCal configuration file you need to remove them. While you're at it, remove the ?> as well so it doesn't happen again! The problem is that this causes the output to the browser to be started before DAViCal has sent it's session cookie. So the browser never receives the session cookie, and never actually manages to log in for more than the single page.
Invitation email with Lightning
For some reasons there is a problem in cooperation between DAViCal and Thunderbird/Lightning. This problem results in some cases that TB/L can't sent out invitation emails. If you add the following line to your configuration <ip-or-canonical-name>-conf.php in /etc/davical you obviate these behaviour:
$c->enable_auto_schedule = false;