Interesting post on the NANOG list about ECN and anycast routing. The summary is that the provider was using ECMP with the ToS byte which means the path selected after connection establishment was different than before because during connection establishment ECT is not set while after connection establishment, ECT is set and CE may be used.
This seems like a big mistake for the provide that configured the entropy calculation for ECMP. They should never have used the ECN bits portion of the ToS byte. Most implementations that I'm aware of use 5-tuple only (IPDA, IPSA, IP Protocol, Src Port, Dst Port).
