Running ASP.NET2.  Every once in a while I get this error:

Unable to cast object of type 'System.Collections.DictionaryEntry' to type
'System.Web.PartitionInfo'.

  at System.Web.PartitionManager.Dispose()
  at System.Web.SessionState.SqlSessionStateStore.OnAppDomainUnload(Object
unusedObject, EventArgs unusedEventArgs)

I setup a partitioned session db by overriding
System.Web.IPartitionResolver and using a simple hash to return an index in
a list of connection strings.

       public String ResolvePartition(Object key)
       {
           String sid = (String)key;

           // hash the incoming session ID into
           // one of the available partitions
           int partitionID = Math.Abs(sid.GetHashCode()) % partitions.Length;

           return partitions[partitionID];
       }

I did it based on the article here:
http://msdn.microsoft.com/msdnmag/issues/05/09/SessionState/default.aspx#S8
and just did the sessiondb instead of the session state server.  As I said
works fine most of the time but throws the above exception once in a while.