I hit a very weird corner case last night.
I use Firefox for Android on my Nexus 6P, and I needed to log into my Google account in order to approve linking YouTube to my Roku device. I also use Google's two-factor authentication with my phone as the key, so when I login on my desktop computer, I hit "OK" to approve the login on the screen that pops up on my phone.
I've done this before, and it's been fine; after I login in Android Firefox, I get the popup and approve it. However, Google has added a new kink to that login. You now need to pick a number that's shown on the browser screen of the device from which you're logging in. But, the device pop-up will cancel if you switch away from it, so the act of switching back to Firefox to read the number cancels the login attempt.
I tried three times to do this, even attempting to use Android 8's split-screen mode, but anything that switched away from the approval pop-up caused it to cancel out.
I ended up reverting to using Chrome on the phone in order to get YouTube linked. Google needs to fix their pop-up mechanism to allow it to work on the device that's requesting the login, possibly by recognizing the same origin and not showing the "number ball" confirmation.