If you move your website from development stage to a live version to new server, lets say from local host or other test server the chances that you will get error 500 are higher. You may encounter this issue especially when accessing K2 or user management component in Joomla! administrator panel.
Some of you may have noticed missing captcha after updating to Joomla! 3 and 2.5 versions despite it was working properly. Bug supposed to be fixed in 3.2.1 release however some may still have problem with this, such as myself when updated to 3.3 version.