I found a little thread about this very same error and it turns out that in Zen Cart v1.3.9d (and previous) when updating the quantity of a product from the shopping cart page the above error is triggered twice in the myDebug log.
I have found out that this was being caused by a line in /includes/classes/shopping_cart.php in function actionUpdateProduct() at about line 1543:
$cart_qty = $this->in_cart_mixed($_POST['products_id']);
Should have read ...
$cart_qty = $this->in_cart_mixed($_POST['products_id'][$i]);
I've edited the shopping_cart.php file accordingly. Initial testing shows that this error log is no longer generating. I don't think I'll need to use that var_dump just yet, but thank you very much for letting me know such a thing exists 🙂 I am sure I'll need it in the near future as I try and erradicate all these error logs!
I'm getting through them. When I first switched to 5.3 my cache folder grew 11GB overnight (error logs). Millions of them! I've now gotten it down to a few KB every few hours. Thanks to forums and helpful advice such as yours.
Now, onto the next one :o