There appears to be a bug in VM 2.9.8a that allows you to add items to the cart that exceed the product's max order level.
If I have an item in my cart and then continue shopping and click "add to cart" on the same product, I see in my system messages "The maximum order level for this product is 1 items." four times, followed by "Product successfully added". My cart then shows that the quantity has indeed increased.
It looks to me like the problem is in /components/com_virtuemart/helpers/cart.php. Line 568 appears to be updating the quantity in the cart before bothering to do checkForQuantities.
Try with 2.9.9 - from http://dev.virtuemart.net/projects/virtuemart/files
Quote from: jenkinhill on August 16, 2014, 15:51:10 PM
Try with 2.9.9 - from http://dev.virtuemart.net/projects/virtuemart/files
Updated to 2.9.9. It still adds the item to the cart. The only difference is that I only see "The maximum order level for this product is 1 items." twice instead of four times.
Update: In 2.9.9, the suspect line is 570:
$cartProductData['quantity'] = $cartProductData['quantity'] + $productData['quantity'];