I think I've found two buggy behaviors.
I'm logged in as a returning customer.
Imagine that, for some reason, I've cleaned up all my addresses through the "Address module" in my customer profile.
I pick a product in the shop, go to the cart.
Curiously, the button checkout is enabled, as you can see on this screenshot! however, I don't have any active address... If I click on "checkout", the order is confirmed
And even more curiously, if I remove the product from the cart, I can always click on the order button! See below:
Hope it helps!