Login system with shopping cart
<p>For exmaple, we've got site that sells car's wheels. I'm not registered there and add wheel to shopping cart (it's stored in <code>cookie</code>). After I login, I get this wheel in my account shopping list.  </p>

<p>What seems wrong: what if someone else has added another wheel from my computer and I'll get it in my shopping list after login.</p>

<p>Can't understand, how to solve this situation.</p>
