This is a collection of tips and common mistakes or errors you may encounter.
You should (almost) never edit the system folder. Any change you want to make to files in system and modules can be made via the cascading filesystem and transparent extension and won't break when you try to update your Kohana version.
Kohana 3 routes are very powerful and flexible, don't be afraid to use as many as you need to make your app function the way you want!
If you get a Reflection_Exception when setting up your site, it is almost certainly because your Kohana::init 'base_url' setting is wrong. If your base url is correct something is probably wrong with your routes.
ReflectionException [ -1 ]: Class controller_<something> does not exist // where <something> is part of the url you entered in your browser
Set your Kohana::init 'base_url' to the correct setting. The base url should be the path to your index.php file relative to the webserver document root.
There is a bug in php which can corrupt your session after a fatal error. A production server shouldn't have uncaught fatal errors, so this bug should only happen during development, when you do something stupid and cause a fatal error. On the next page load you will get a database connection error, then all subsequent page loads will display the following error:
ErrorException [ Notice ]: Undefined index: id MODPATH/orm/classes/kohana/orm.php [ 1308 ]
To fix this, clear your cookies for that domain to reset your session. This should never happen on a production server, so you won't have to explain to your clients how to clear their cookies. You can see the discussion on this issue for more details.