Before you start creating a new application.master set CustomErrors="Off" and .. in the web config. Then you will get "developer friendly" error messagess.
Mistakes that I've made that kept me rather busy:
- Place the placeholder "PlaceHolderUtilityContent" AFTER the ending FORM-tag tag. If not the post backs stops working
- Forgot to create a HEAD-section similiar to the original application.master
- Missing place holders are an obvious error source