One of our clients is seeing these errors related to users, and appearing to be in part due to Coveo (Sitecore 8.1 Update 1, Coveo June 2016 instance with on-prem free version).
In the first two errors, referencing "GetUserMembers", the report is these are Sitecore user accounts they created but then deleted due to usernames they didn't like (usernames masked to protect the guilty). The third I've seen on another project when I tried to re-index the Coveo_master_index, but this is just cropping up during normal usage.
I know this isn't too much to go on, but I was wondering if anyone had insight to the errors and if there were resolutions, or if this is just normal and people shouldn't be obsessing over the Sitecore logs?
6420 00:00:49 ERROR An error occurred while calling method "GetUserMembers".
Exception: Coveo.Connectors.Sitecore2.SitecoreWebServiceExceptions.SitecoreWebServiceUserNotFoundException
Message: The user "sitecore\[USERNAME]" does not exist.
Source: Coveo.Connectors.Sitecore2.SitecoreWebService
at Coveo.Connectors.Sitecore2.SitecoreWebService.Wrapper.BaseSitecoreWrapper.GetUserMembers(String p_UserName)
at Coveo.Connectors.Sitecore2.SitecoreWebService.SitecoreWebService.<>c__DisplayClass1d.<GetUserMembers>b__1c()
at Coveo.Connectors.Sitecore2.SitecoreWebService.SitecoreWebService.TryCatchWrapper[T](Func`1 p_Action, String p_MethodName)
16128 00:00:49 ERROR An error occurred while calling method "GetUserMembers".
Exception: Coveo.Connectors.Sitecore2.SitecoreWebServiceExceptions.SitecoreWebServiceUserNotFoundException
Message: The user "sitecore\[USERNAME]" does not exist.
Source: Coveo.Connectors.Sitecore2.SitecoreWebService
at Coveo.Connectors.Sitecore2.SitecoreWebService.Wrapper.BaseSitecoreWrapper.GetUserMembers(String p_UserName)
at Coveo.Connectors.Sitecore2.SitecoreWebService.SitecoreWebService.<>c__DisplayClass1d.<GetUserMembers>b__1c()
at Coveo.Connectors.Sitecore2.SitecoreWebService.SitecoreWebService.TryCatchWrapper[T](Func`1 p_Action, String p_MethodName)
8772 00:00:30 ERROR An error occurred while calling method "GetUsersForDomain".
Exception: System.ArgumentException
Message: Item has already been added. Key in dictionary: '[USERNAME]' Key being added: '[USERNAME]'
Source: mscorlib
at System.Collections.Hashtable.Insert(Object key, Object nvalue, Boolean add)
at System.Web.Security.MembershipUserCollection.Add(MembershipUser user)
at System.Web.Security.SqlMembershipProvider.FindUsersByName(String usernameToMatch, Int32 pageIndex, Int32 pageSize, Int32& totalRecords)
at Sitecore.Security.SwitchingMembershipProvider.<>c__DisplayClass4.<FindUsersByName>b__3(MembershipProviderWrapper wrapper, Int32 index, Int32 count, Int32& providerRecords)
at Sitecore.Common.PagingHelper`2.GetInputRecords(TInput input, GetElements getElements, Int32 startRecord, Int32 recordCount, Int32& totalRecords)
at Sitecore.Common.PagingHelper`2.GetRecords(Int32 pageIndex, Int32 pageSize, IEnumerable`1 inputList, GetElements getElements, Int32& totalRecords)
at Sitecore.Security.SwitchingMembershipProvider.GetUsers(MembershipProviderWrapperList wrappers, Int32 pageIndex, Int32 pageSize, Int32& totalRecords, GetElements getElements)
at Sitecore.Security.SwitchingMembershipProvider.FindUsersByName(String userNameToMatch, Int32 pageIndex, Int32 pageSize, Int32& totalRecords)
at Sitecore.Security.SitecoreMembershipProvider.<>c__DisplayClass12.<FindUsersByName>b__11()
at Sitecore.Data.DataProviders.NullRetryer.Execute[T](Func`1 action, Action recover)
at Sitecore.Security.SitecoreMembershipProvider.FindUsersByName(String userNameToMatch, Int32 pageIndex, Int32 pageSize, Int32& totalRecords)
at System.Web.Security.Membership.FindUsersByName(String usernameToMatch, Int32 pageIndex, Int32 pageSize, Int32& totalRecords)
at Sitecore.Security.Domains.Domain.GetUsersByName(Int32 pageIndex, Int32 pageSize, String search, Int32& total)
at Sitecore.Security.Domains.Domain.GetUsers(Int32 pageIndex, Int32 pageSize, Int32& total)
at Sitecore.Security.Domains.Domain.GetUsers(Int32 pageIndex, Int32 pageSize)
at Coveo.Connectors.Sitecore2.SitecoreWebService.Wrapper.BaseSitecoreWrapper.GetUsersForDomain(String p_DomainName, Int32 p_PageIndex, Int32 p_PageSize)
at Coveo.Connectors.Sitecore2.SitecoreWebService.SitecoreWebService.<>c__DisplayClass1a.<GetUsersForDomain>b__19()
at Coveo.Connectors.Sitecore2.SitecoreWebService.SitecoreWebService.TryCatchWrapper[T](Func`1 p_Action, String p_MethodName)