Collection was modified; enumeration operation may not execute

I cannot get to the bottom of this error because it happens only in one instance, and I can't find any code that can be the cause of the error. I have a 3.5 web service that I'm calling from a multi-threaded, CAB client. I have a bunch of unit tests against the web service (from both 3.5 and 2.0 code), and it works fine. However, in the actual application, it doesn't work 90% of the time and, the remaining 10% of the time, it decides to work. The code: ``` Friend Function ExecuteSearch(ByVal query As String) As List(Of SomeObject) Dim searchResults As List(of Object) = _searcher.UserSearch(query) Return searchResults End Function // In Searcher Public Function UserSearch(ByVal query As String) As List(Of SomeObject) Return Translate(Search.GetResults(query)) End Function // In Search Public Function GetResults(ByVal query As String) As List(Of SomeObject) Dim service As New FinderService.FinderService() Dim results As New List(Of String) Dim serviceResults As IEnumerable(Of String) = service.Search(query) // <-- ERRORS OUT HERE results.AddRange(serviceResults) Return results End Function // In the service Public Function Search(ByVal query As String) As IEnumerable(Of String) Initialize() // Initializes the _accounts variable Dim results As New List(of String) For Each account As User In _accounts If a bunch of conditions Then results.Add(account.Name) End IF End For Return results End Function ``` The breakpoints hit these codes (in this order). The line that errors out is in the "GetResults" method. Any help would be appreciated.