The timeout period elapsed prior to obtaining a connection from the pool

If you are working with .NET applications, at some time you might get the below (or similar) error message: 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.

The most probable reason you are getting the above error message, is that your application opens connection to the database without closing them. This situation is then repeated and after a number of times, the application throws an exception because it cannot open more than a given number (max pool) of connections to the database.

using (SqlConnection conn = new SqlConnection(connString))
   {
      SqlCommand cmd = new SqlCommand("SELECT @@VERSION", conn);
      conn.Open();      
      SqlDataReader dr = cmd.ExecuteReader();
      //process the result
      dr.Close();
   }
}