In SharePoint timer job history I have noticed that there's the error in starting User Profile synchronization:

Could not connect to http://spsrv:5725/ResourceManagementService/MEX. TCP error code 10061: No connection could be made because the target machine actively refused it <ip-adress:port>. (read more...)

When user tells you that he has got an error using SharePoint portal service, you ask him about the page he was, which buttons he clicked and, if user hasn’t still closed browser, ask to send you screenshot of the error. It contains very useful information – CorrelationId. (read more...)