Snippet from public announcement: 

" Based on past experiences and comments on the mailing lists, error handling was a bit deficient from the configuration file. Log files could get full of error messages when SIP messages were incorrect, the processing continued even it should have stopped.

With the new framework, the administrator can write a error_route block and there decide what to do when an error is encountered. Right now, error_route is executed for parsing errors in sip requests - one can send back now the appropriate reply. "

See full announcement:

http://openser.org/pipermail/devel/2007-January/005237.html