There are a number of situations in the SIP protocol (RFC 3261 & extensions) where behaviour is allowed, but underspecified. While some of these are edge cases, others are just due to just not having any defined behaviour or not defined well enough.
There are also a number of situations where situations have needed clarification, and as such i'm pointing them out by adding them here.
Note: i'm adding these as i come across them, so it's far from complete - please feel free to add - (if you're not a voip.co.uk customer, email me for a login)
RFC 3261 states that UA's s should re-transmit the request using the contact header as the new R-URI. This does not mean the To or From header should change, only the R-URI.
Proxies should treat a REGISTER redirect in exactly the same way as it would treat any other method that generates a 302.
How this interacts with route headers (specifically route-record and service-route) is not specified. Add outbound draft into the mix and it's totally un-interoperable right now.