It's often a change in interface. If you build from source, old code simply won't compile with new incompatible interface, and won't get to the testing stage.
The impact comes from the dependencies you are using.
Depending how the software is made, it can be noticed early on build time.
But it can also be noticed just on runtime, e.g. dynamic libraries, which is hopefully noticed on testing stage.
However, how it impacts downstream, the dependents of your software, cannot be tested. You can only inform about it. For people, with changelog.
For automated systems, with version number. But if you do not follow systematic version numbering, you fail to inform automatic systems. They update to the later version of you software and dependents will break.
However, how it impacts downstream, the dependents of your software, cannot be tested. You can only inform about it. For people, with changelog. For automated systems, with version number. But if you do not follow systematic version numbering, you fail to inform automatic systems. They update to the later version of you software and dependents will break.