1. Create a lobby with a server far away from you
2. Pick any hero
3. Fill 5 out of 6 of your item slots
4. Move to the secret or sidelane shop
5. Try to buy an item, but spam its button
6. More realistic scenario, have multiple items in quick-buy which are available at those shops and spam the quickbuy button

Result: If your delay is big enough, you may buy more than 1 item and the others land on the ground due to full inventory. You don't get the "inventory is full" error message. The bigger your delay is, the more likely this is going to happen.

Expected: The "inventory is full" error should be server-sided, not client-sided, so the game sees instantly that you are full after the 1st bought item and doesn't allow buying more items from side lane or secret shops.