When reading the terms of service, I saw :
Google does not guarantee that Registrant will be able to register or renew a desired domain name, even if Google’s systems indicate that the domain name is available or Registrant is able to complete an order with respect to such domain name.
Registrar shall give no guarantee for a successful allocation of ordered domains and becomes free from the duty to perform the service in case of impossibility of the order.
Will I get an error messages in the webapp if I order a domain which isn't available, or will it silently fail (by silently fail I mean the website still claims I own the domain because the order was successful) ?
I can't test it since no bank allow to open accounts in foreign currencies for individuals in my country (the foreign currency being $)
If it silently fails, then it would confirm a server security vulnerability (I would be able to take ownership of the google.com domain in Webmaster Tools).