I bought a new phone and am thus transferring my google authenticator entries to the new phone. This showed me some important things that might be well-known to the most of you but were either new to me or became highlighted by the process again:
Transferring your phone's entries to a new phone doesn't transfer the keys as well. I was actually told otherwise. So if you just use the google function to play the backup of your old phone on the new one, the authenticator should be empty. This means you actually have to make backups.
Write your codes down. The first time I started using it, I was too lazy for that. The first transfer by phone worked by me simply deactivating 2SA and then activating it with the new phone. In case the old one breaks or gets stolen, this would be disastrous. So write down your codes!
When activating a new code, write it down first and then enter it by using your own notes, not the QR-Code on the screen. This way you make sure that your written down code actually is correct and that you can read it. My handwriting is bad enough that doing this with roughly 20 accounts, I actually got it wrong twice! You know, those O and 0, these 2 and Z, those S and 5 and those G and 6...
By writing it down first and then using the notes to activate, you can be sure that you'll be able to replicate the process again.