I get the following error if I try to log in as user1 with a wrong password and then as user2 with a correct password.

2015-10-15 09:05:58,605 ERROR [org.keycloak.authentication.AuthenticationProcessor] (default task-24) failed authentication: USER_CONFLICT: org.keycloak.authentication.AuthenticationFlowException

at org.keycloak.authentication.AuthenticationProcessor.setAutheticatedUser(AuthenticationProcessor.java:203) [keycloak-services-1.6.0.Final-SNAPSHOT.jar:1.6.0.Final-SNAPSHOT]

at org.keycloak.authentication.AuthenticationProcessor$Result.setUser(AuthenticationProcessor.java:332) [keycloak-services-1.6.0.Final-SNAPSHOT.jar:1.6.0.Final-SNAPSHOT]

I think the reason for that is the context.setUser(user) call in the AbstractUsernameFormAuthenticator.validateUser method.