Not necessarily a piece of code, so much as a decision on a backend service.
2 years ago when I switched from MySQL to Firebase, I went all in. I moved everything to Firebase. For a while, it was great, but as usual, 3rd party services make (breaking) changes and have their own problems. Now, I'm backing away from Firebase and going back to MySQL. I'll keep Firebase around for "some" non-mission critical features, but the majority of the system is slowly being re-written in PHP / MySQL, again.
The website is just to important to me to leave it in the hands of a proprietary 3rd party service that I can't move to a different server / stop a feature from being implemented or whatever.