Application suspended
If the OAuth App you set up has been suspended (due to reported abuse, spam, or a mis-use of the API), GitHub will redirect to the registered callback URL using the following parameters to summarize the error:
http://your-application.com/callback?error=application_suspended
&error_description=Your+application+has+been+suspended.+Contact+support@github.com.
&error_uri=/apps/building-integrations/setting-up-and-registering-oauth-apps/troubleshooting-authorization-request-errors/%23application-suspended
&state=xyz
To solve issues with suspended applications, please contact your site administrator.
Redirect URI mismatch
If you provide a redirect_uri that doesn't match what you've registered with your application, GitHub will redirect to the registered callback URL with the following parameters summarizing the error:
http://your-application.com/callback?error=redirect_uri_mismatch
&error_description=The+redirect_uri+MUST+match+the+registered+callback+URL+for+this+application.
&error_uri=/apps/building-integrations/setting-up-and-registering-oauth-apps/troubleshooting-authorization-request-errors/%23redirect-uri-mismatch
&state=xyz
To correct this error, either provide a redirect_uri that matches what you registered or leave out this parameter to use the default one registered with your application.
Access denied
If the user rejects access to your application, GitHub will redirect to the registered callback URL with the following parameters summarizing the error:
http://your-application.com/callback?error=access_denied
&error_description=The+user+has+denied+your+application+access.
&error_uri=/apps/building-integrations/setting-up-and-registering-oauth-apps/troubleshooting-authorization-request-errors/%23access-denied
&state=xyz
There's nothing you can do here as users are free to choose not to use your application. More often than not, users will just close the window or press back in their browser, so it is likely that you'll never see this error.