Often you can be certain that you know what tp expect from your own code, but not from dependencies or external systems. So checking that your assumptions about them are right is a major reason to run and rerun your code.
That is good reason to minimize amount of dependencies and only use the ones you know and can reason about. It is also part of what I do to help me reason about code before I execute it.
As I mentioned, if I don't understand a dependency or external system I make separate toy project where I can rapidly experiment and learn.
Think of it as having fun in a aircraft simulator. You play with it so that you are prepared to fly the actual plane.
Also checking your assumptions by trying to see if the code works is a problem in itself. A lot of these broken assumptions will not break your code immediately but maybe sometime later. Maybe when your application is under load or maybe when clock on the server is moved back by an hour or maybe when the connection breaks in a certain way.
Base your work on knowing how something works and not assuming.
Best way to limit the risk of your application failing due to broken assumption is to limit your reliance on assumptions in the first place.
