What makes a DataStore fail?
You can read a complete list of DataStore errors and why they occur here, although most of these are due to user error.
If you're wondering about why the DataStores themselves fail from time to time, it's a pretty simple answer. Sometimes, things just go wrong!
Maybe a packet sent over the network from your computer to the server where the data is hosted got dropped?
Or maybe an unknown bug in the DataStore software caused data to be corrupted?
Or maybe someone in the server room tripped over a cord and pulled the plug on the server that was handling your request?
There are an infinite number of things that can cause networks to fail, and sometimes the planets align just right and you get to be the lucky winner who gets to be the butt end of the bug. The best (and in most cases, only) thing you can do is to write code "defensively". Use
pcall() to ensure that the result of a network function call came back OK! Or write more robust code to handle an unusual number of subsequent failed attempts (see Handling DataStore failures)! At the end of the day, you can't ensure that someone else's code or network will work, but you can do your best to make sure yours does.
A data store is an external service which will be running on its own servers. Though I do not know the internal setup. This will require a "handshake" to take place as we are sending data from one system to another.
Though this data should be validated before sending, much like FE it will be validated again by the receiver before it can then be stored. Then a response can be sent back to say that the data has been saved.
So what can go wrong?
This list does not include errors caused by the programmer by not using the server as it is intended to be used.
I hope this helps. Please comment if you want me to expand upon any of the points.