Depending on your system, connections can be a significant issue with MARS, where sql's connection limit can get hit and the pooler restarts, which will cause some failed logins. It's very frustrating to troubleshoot from the DBA end as you can watch low traffic from sys.dm_exec_requests/sys.dm_exec_sql_text, but get high counts from sys.dm_exec_connections, where when joined to sys.dm_exec_sessions it looks like your web service account has thousands of connections for minimal sql because MARS will create a connection for each statement in the batch, but the above DMVs only show the specific query running. SP_who2 won't show all the logins mapped either. It's REALLY annoying.