"DB_DataObject Error: DB Error: connect failed"
If you get this error on a new Drupal/CiviCRM installation, you may have skipped the step of running CiviCRM's installation script. You can not install CiviCRM using the standard Drupal module installer. Review the installation instructions here.
"Could not find valid Key"
If you are getting this error when submitting a form (search or adding / editing records), check the following:
"Failed to initialize storage module: user" fatal error message (new installs)
If you see this error, you may need to modify the session.save_handler method for your site. Check with your hosting provider for the recommended way to do this. More info at this forum post.
Using the Built-in Debugging Tools
CiviCRM provides several URL-param debug settings which can help debug and resolve problems such as corrupt sessions and obsolete template caches. In order to use these URL parameters, you must first enable debugging from Administer CiviCRM » Global Settings » Debugging.
Resolving RELAY-DENIED Errors When Trying to Send Emails to Contacts
Configuring PHP to Handle Import Files from/on Macintosh Computers
In order for CiviCRM to properly handle CSV import files created on Macs, you may need to update a PHP setting for detecting line endings. The symptom of this problem is that the "Import Contacts: Match Fields" step shows the file as one big record - rather than recognizing the end of each row.
Database version inconsistencies / upgrade problems
Download and run Database Troubleshooting Tools to test the current state of the database and provides a diagnosis. The tools suite also includes a repair facility.
Windows XP SP3: Problem setting the /sites/default/files directory permissions so it is writable
There is a weird glitch that can happen under Windows XP (SP3) (and perhaps other Windows OS versions): if you try to change the permissions for a file or directory, they won't 'stick.' For example, if you go to your <yoursite>/sites/default/files directory and right click on it and bring up the Properties dialog, you can see the permissions for the directory. If this is set to read only, you will get an error message when you install CiviCRM telling that it needs to be writable. If you click on the "Read Only" checkbox so that it is not checked and then OK your change, the permission should be changed. (The directory should now be writable.) However, sometimes this change will not actually take effect. You have to make the change using the ATTRIB command in a command prompt window. Open a command prompt window and go to the <yoursite>/sites/default directory. Type "help attrib" to get help using the attrib command and then enter the appropriate arguments to set the /files directory permissions (attributes) so that it's writeable. This will usually invovle the "-R" attribute and the "/D" argument.