Hi to all.
Probably i catched a bug in the keycloak authentication flow.
This is my user case:
Configuration:
1) I've created a new realm, say "TestRealm"
2) I've created 1 role: "testRole"
3) I've created 2 users: "userTest1" and "userTest2"
4) In the role mapping tab of each user i've assigned "testRole" to both of them
5) In the credential tab of each user i've changed their pwd

Use case:
1) I try to access the account application from: https://localhost:8444/auth/realms/TestRealm/account/
2) I insert username: userTest1
                pwd: (a wrong password)

Login page displays a tooltip saying "invalid username or password"

3) Withouth any page refreshing i try to login again with second user:
             username: userTest2:
             pwd: (whatever right or wrong password)

Keycloak catch an exception:
The page displays:
                                We're sorry ...
                                Invalid username or password.
                                 << Back to Application

Keycloak console displays this exception:
13:35:27,343 WARN  [org.keycloak.events] (default task-62) type=LOGIN_ERROR, realmId=44cefb3e-1b9e-4eb0-9cfe-267e0153b0de, clientId=account, userId=5c9afd4e-74f4-4c51-9015-d9d4a7ef883f, ipAddress=127.0.0.1, error=invalid_user_credentials, auth_method=openid-connect, auth_type=code, redirect_uri=https://localhost:8444/auth/realms/PROVA/account/login-redirect, code_id=2920658d-1137-4caa-a2a2-0c530555b81d, username=userTest
13:35:33,818 ERROR [org.keycloak.authentication.AuthenticationProcessor] (default task-72) failed authentication: USER_CONFLICT: org.keycloak.authentication.AuthenticationFlowException
at org.keycloak.authentication.AuthenticationProcessor.setAutheticatedUser(AuthenticationProcessor.java:203)
at org.keycloak.authentication.AuthenticationProcessor$Result.setUser(AuthenticationProcessor.java:332)
at org.keycloak.authentication.authenticators.browser.AbstractUsernameFormAuthenticator.validateUser(AbstractUsernameFormAuthenticator.java:129)
at org.keycloak.authentication.authenticators.browser.UsernamePasswordForm.validateForm(UsernamePasswordForm.java:41)
at org.keycloak.authentication.authenticators.browser.UsernamePasswordForm.action(UsernamePasswordForm.java:34)
at org.keycloak.authentication.DefaultAuthenticationFlow.processAction(DefaultAuthenticationFlow.java:62)
at org.keycloak.authentication.DefaultAuthenticationFlow.processAction(DefaultAuthenticationFlow.java:54)
at org.keycloak.authentication.AuthenticationProcessor.authenticationAction(AuthenticationProcessor.java:692)
at org.keycloak.services.resources.LoginActionsService.processFlow(LoginActionsService.java:307)
at org.keycloak.services.resources.LoginActionsService.processAuthentication(LoginActionsService.java:288)
at org.keycloak.services.resources.LoginActionsService.authenticateForm(LoginActionsService.java:334)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.jboss.resteasy.core.MethodInjectorImpl.invoke(MethodInjectorImpl.java:137)
at org.jboss.resteasy.core.ResourceMethodInvoker.invokeOnTarget(ResourceMethodInvoker.java:296)
at org.jboss.resteasy.core.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:250)
at org.jboss.resteasy.core.ResourceLocatorInvoker.invokeOnTargetObject(ResourceLocatorInvoker.java:140)
at org.jboss.resteasy.core.ResourceLocatorInvoker.invoke(ResourceLocatorInvoker.java:103)
at org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:356)
at org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:179)
at org.jboss.resteasy.plugins.server.servlet.ServletContainerDispatcher.service(ServletContainerDispatcher.java:220)
at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:56)
at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:51)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at io.undertow.servlet.handlers.ServletHandler.handleRequest(ServletHandler.java:86)
at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:130)
at org.keycloak.services.filters.KeycloakSessionServletFilter.doFilter(KeycloakSessionServletFilter.java:59)
at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:60)
at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:132)
at io.undertow.servlet.handlers.FilterHandler.handleRequest(FilterHandler.java:85)
at io.undertow.servlet.handlers.security.ServletSecurityRoleHandler.handleRequest(ServletSecurityRoleHandler.java:62)
at io.undertow.servlet.handlers.ServletDispatchingHandler.handleRequest(ServletDispatchingHandler.java:36)
at org.wildfly.extension.undertow.security.SecurityContextAssociationHandler.handleRequest(SecurityContextAssociationHandler.java:78)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at io.undertow.servlet.handlers.security.SSLInformationAssociationHandler.handleRequest(SSLInformationAssociationHandler.java:131)
at io.undertow.servlet.handlers.security.ServletAuthenticationCallHandler.handleRequest(ServletAuthenticationCallHandler.java:57)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at io.undertow.security.handlers.AbstractConfidentialityHandler.handleRequest(AbstractConfidentialityHandler.java:46)
at io.undertow.servlet.handlers.security.ServletConfidentialityConstraintHandler.handleRequest(ServletConfidentialityConstraintHandler.java:64)
at io.undertow.security.handlers.AuthenticationMechanismsHandler.handleRequest(AuthenticationMechanismsHandler.java:58)
at io.undertow.servlet.handlers.security.CachedAuthenticatedSessionHandler.handleRequest(CachedAuthenticatedSessionHandler.java:72)
at io.undertow.security.handlers.NotificationReceiverHandler.handleRequest(NotificationReceiverHandler.java:50)
at io.undertow.security.handlers.SecurityInitialHandler.handleRequest(SecurityInitialHandler.java:76)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at org.wildfly.extension.undertow.security.jacc.JACCContextIdHandler.handleRequest(JACCContextIdHandler.java:61)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at io.undertow.servlet.handlers.ServletInitialHandler.handleFirstRequest(ServletInitialHandler.java:282)
at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:261)
at io.undertow.servlet.handlers.ServletInitialHandler.access$000(ServletInitialHandler.java:80)
at io.undertow.servlet.handlers.ServletInitialHandler$1.handleRequest(ServletInitialHandler.java:172)
at io.undertow.server.Connectors.executeRootHandler(Connectors.java:199)
at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:774)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

13:35:33,819 WARN  [org.keycloak.events] (default task-72) type=LOGIN_ERROR, realmId=44cefb3e-1b9e-4eb0-9cfe-267e0153b0de, clientId=account, userId=null, ipAddress=127.0.0.1, error=invalid_user_credentials, auth_method=openid-connect, auth_type=code, redirect_uri=https://localhost:8444/auth/realms/PROVA/account/login-redirect, code_id=2920658d-1137-4caa-a2a2-0c530555b81d, username=userTest2


I experienced this error while debugging my custom user federation provider. So i tried to replicate it with a clean situation like described in the use case above.
Debugging my userfederation provider i could realize the real authentication flow:

When userTest1 logs in the flow starts from:

UsernamePasswordForm.action() ---> validateUser --->  ----> UserFederationProvider.isValid() ----> ... ... ... ---> UsernamePasswordForm.validatePassword() ----> authenticate 

When userTest2 logs in after userTest1 failure the flow starts from the UserFederationProvider.isValid():

UserFederationProvider.isValid() (the AuthenticationFlowContext user is still userTest1 )---> ... ----> UsernamePasswordForm.action() ---> validateUser --->  ----> UserFederationProvider.isValid() ----> ... ... ... --->Exception on Context.set(user).
It seems like Context is not cleaned after the first wrong login attempt, bringing with itself the userTest1 user object on the second one. So when keycloak tries to set the new user object catches a USERCONFLICT exception.