When I log into Gmail with 2 factor authentication, I enter my username and password and get taken to a page asking me to enter a security code sent to a phone. If I enter a backup code into this form Gmail recognizes that I entered a backup code and says:
It looks like you entered a backup code. Click "Try another way to sign in" and select the backup code option.
Is there a security reason for Gmail to require me to go to a separate page to enter a backup code? The only reason I can think of is so that someone doesn't accidentally use the last backup code and get locked out, but it's hard to believe someone could enter a backup code without realizing it.