See http://dotnetdebug.net/2007/04/04/systeminvalidoperationexception-timeout-expired-the-timeout-period-elapsed-prior-to-obtaining-a-connection-from-the-pool-this-may-have-occurred-because-all-pooled-connections-were-in-use-and-max/

or

http://www.15seconds.com/issue/040830.htm

>I am not sure if this is a .NET error or a SQL Server error. It happened when three connections were trying to access a Web Service.
>
>System.InvalidOperationException: Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached.
>
>Anyone would know more about it? If yes, do you have any recommendation on how to avoid it?