Gotchas

If you try to override template_preprocess_user_profile(&$variables)  with phptemplate_preprocess_user_profile(&$variables) the function will NOT be called unless you copy /user-profile.tpl.php in your template directory.

Not 'taking' login and no error message.  If you have restored a whole site check the $cookie_domain entry in settings.php.  If it is not the same as the URL your browser is denying cookies for the domain.

For some reason most of the times the cck data is returned as a serialized string.
When theming you want to unserialize the data so you can access the individual fields.
if(!is_array($item['data'])) $item['data'] = (array) unserialize($item['data']);

The user has "edit any page content" permission but can't see the edit tab.
Check if the user's role can use the page Input format in admin/settings/filters

In Drupal 6 you added a template in your theme directory but you see no change.
Empty the cache since all the templates are cached in the theme registry. Use the develop module to check if the newly added template is in the Theme registry

Not updating exposed view fields in a custom content type.
You have a module defining a content type and a file my.module.views.inc  After ypu update the .inc module you don;t see the changes when you build a view.  You need to check "Disable views data caching"in the Tools tab of the Views UI or manually clear the table cache_views 

delete FROM cache_views 

Clean URL not working even if mod_rewrite is enabled.  
The default apache configuration prevents clean url from working.  Modify as follows.

root@domU-12-31-38-07-4D-3A:/etc/apache2/sites-available# cat default
<VirtualHost *:80>
   ServerAdmin webmaster@localhost

   DocumentRoot /var/www/dev/drupal
   <Directory /var/www/dev/drupal>
      Options +Indexes
      allow from all
   </Directory>

   ErrorLog /var/log/apache2/error.log

   # Possible values include: debug, info, notice, warn, error, crit,
   # alert, emerg.
   LogLevel warn

   CustomLog /var/log/apache2/access.log combined

</VirtualHost>