The app I work on has some undocumented but consistent behavior when filtering and sorting a queue.
The formal requirement is that, if the filter returns more than 1000 results, a warning message is displayed and only 1000 results are shown.
As an implementation detail, it so happens that the app in fact displays the first 1000 results (as determined by the sort order the user had specified).
This is not documented anywhere and was never intended as a guarantee; it was just a consequence of the specific implementation. But it has worked this way consistently for many years.
To improve database performance, the devs would like to modify their implementation of this feature in a way such that this would no longer be true; users would get 1000 matching results but there would be no predictable pattern to which 1000 they get.
My current but untested intuition is that users are not aware of the exact current behavior and do not rely on it, so it would not upset anyone if it suddenly changed.
But I’m not really sure how to test this assumption. I don’t want to explain the current behavior to users because I’m trying to find out whether they already know about it; it is easy to say “yes, I knew that” after I’ve already explained it. But I don’t know how to find out if anyone relies on something without explaining what it is!