Something that just cropped up today…
As your site grows, and you add more and more config to the app.yml file, be very careful not to re-declare the same named element. Symfony will not warn you, and there will be no errors – but the second (lower) instance will simple overwrite the first!
For example, if you have forgotten that you already have an element called profile hiding near the top of the file, and you add another one called profile at the bottom, with all your profile page settings – the lower one will destroy the one higher in the file.
This can be hard to spot if you have been meticulously coding and added a default value to all your sfConfig::get() calls because chances are when you are developing the site, all your defaults will be the same as the ones you have specified in app.yml.
The problem will become apparent when an end user tries to change the configuration and the change is ignored, because Symfony is simply using the default value rather than the one the user thinks they are setting.
Careful out there kids!