To reproduce:

Go to https://demo.stack-auth.com/
Hit sign in or sign up (bug happens with either, but I'll hit sign in for these steps).
Hit "Sign in with Google" or "Sign in with GitHub" (bug happens with either, but I'll hit "Sign in with Google").
Choose an account.
In the screen titled "Sign in to stack-auth.com" or "You’re signing back in to stack-auth.com", hit Cancel.
It should say "OAuth provider access denied" and "The sign-in operation has been cancelled or denied. Please try again."
Hit "Sign in again".
Hit "Sign in with Google" or "Sign in with GitHub".
Choose the same account or even a different account.
In the screen titled "Sign in to stack-auth.com", hit Continue.
If it says "OAuth provider access denied" again, then you encountered the bug. It obviously should not says access denied anymore.
If you then click "Go Home", you will see that you are actually signed in. But that is not good user experience and new users might leave before trying that.

Users may hit cancel by accident or because they decide they want to sign in with a different account. So a fix for this should be prioritized.