Some details on how this assumption was made would be nice, but I think it's pretty obvious that any developer involved in a project can make a reasonable assumption of how rare a bug is depending on the technical details on what is required for the bug to happen. For example, if we say for the sake of argument that a hypothetical bug requires you to have more than ten contacts of the exact same name and these also need to share the same country and area code, one can make the assumption this use case is very rare without knowing the exact number of users that this applies to, just based on common sense regarding how the application is normally used.
edit: The linked github issue says:
> The TL;DR is that if someone had conversation trimming on, it could create a rare situation where a database ID was re-used in a way that could result in this behavior. It was very difficult to track down, with earlier phases involving getting additional logging into builds. Once we had some more information, it did in fact become our top priority, a fix was made, and we got it out as quickly and as safely as possible. The fix itself should make it so that database issues like the one that caused this bug can't happen again.
edit: The linked github issue says:
> The TL;DR is that if someone had conversation trimming on, it could create a rare situation where a database ID was re-used in a way that could result in this behavior. It was very difficult to track down, with earlier phases involving getting additional logging into builds. Once we had some more information, it did in fact become our top priority, a fix was made, and we got it out as quickly and as safely as possible. The fix itself should make it so that database issues like the one that caused this bug can't happen again.