There is an error_log object in a zope server's root folder. This is a useful way to see details of recent errors.
You can also check a box to send tracebacks to the event.log. Note the traceback in error_log is sometimes more detailed, eg with page template errors.
Plone sites also have their own error_log, and a user interface in plone setup. Presumably an error_log shows all errors that occur below it and aren't handled by an error_log further down.