Yes yes all zones mixes coaches. Since the discussion was on SER, so i gave SER example.
Yes SER Humsafar spare rake got all mixed because after NRC attack by mob, the rake parked at Uluberia station was fully damaged. The rake was sent to KGPW and got repaired. But all coaches didn't come out from KGPW together, they came back part by part as got repaired. So, the rake later formed mixed with existing spare coaches outside with coaches that came phasewise from Workshop. So a mixed rake now. Then usual reshuffling goes on in SER with all coaches.