When opening a connection to SQL Server 2005 from our web app, we occasionally see this error:

"Impersonate Session Security Context" cannot be called in this batch because a simultaneous batch has called it.

We use MARS and connection pooling.

The exception originates from the following piece of code:

protected SqlConnection Open()
{
    SqlConnection connection = new SqlConnection();
    connection.ConnectionString = m_ConnectionString;
    if (connection != null)
    {
        try
        {
            connection.Open();
            if (m_ExecuteAsUserName != null)
            {
                string sql = Format("EXECUTE AS LOGIN = {0};", m_ExecuteAsUserName);
                ExecuteCommand(connection, sql);
            }
        }
        catch (Exception exception)
        {
            connection.Close();
            connection = null;
        }
    }
    return connection;
}

I found an MS Connect article which suggests that the error is caused when a previous command has not yet terminated before the EXECUTE AS LOGIN command is sent. Yet how can this be if the connection has only just been opened?

Could this be something to do with connection pooling interacting strangely with MARS?

UPDATE: For the short-term we have implemented a workaround by clearing out the connection pool whenever this happens, to get rid of the bad connection, as it otherwise keeps getting handed back to various users. (Not too bad as this only happens a couple of times a day.) But if anyone has any further ideas, we are still looking out for a real solution...