INVITE is proxied and forked by something downstream.
Several 1xx responses are received with different to-tags
A 2xx is received.
What happens to the derived sessions with to-tags that don't match the one in the 2xx? It's not clear from the standard who should invalidate them. Since there is also the possibility of a race condition at the downstream proxy where you could get multiple 2xx responses it's not clear that invalidating all the other sessions except the one for which you get the 2xx is the right thing to do.
The other case is receiving only final error responses because its not clear from the standard if its OK at that point to go ahead and invalidate the associated derived sessions.