An example is, say you have a counter value. There’s two types of operations users can make - either reset the counter to some value or increment it. The counter was 20 and now it’s 25. Did the user set the counter to 25 or did they increment it 5 times? When there’s only one editor, it doesn’t matter - the result is 25 no matter what. But if two users both edited the value at the same time (the other user changed 20 to 30) now we have two different results based on the users intent. Either the new value be 35 (20+5+10) or we should pick one of the results (25 or 30) and just converge to that new value.

You can’t tell what the users intent was by simply diffing the old and new contents. The right approach is to capture the users intent directly from the software that they use to edit the value, and then preserve that intent through the synchronisation system.

This problem is also easy to reproduce with edits on lists / strings which contain repeated elements.