title: When does "SqlConnection does not support parallel transactions" happen?
body: <p>I have a ton of rather working code that's been here for months and today I saw the following exception logged:</p>
  
  <pre><code>System.InvalidOperationException
  SqlConnection does not support parallel transactions.
      at System.Data.SqlClient.SqlInternalConnection.BeginSqlTransaction(
         IsolationLevel iso, String transactionName)
      at System.Data.SqlClient.SqlConnection.BeginTransaction(
         IsolationLevel iso, String transactionName)
      at my code here
  </code></pre>
  
  <p>and I'd like to investigate why this exception was thrown. I've read MSDN description of <code>BeginTransaction()</code> and all it says is that well, sometimes this exception can be thrown.</p>
  
  <p>What does this exception mean exactly? What is the deficiency in my code that I should be looking for?</p>