In engineering or other fields, the mentality “it can’t happen” could do harms to the project. This becomes prevalent in the software engineering and IT profession as well.
If, for example, you have stumbling across the application codebase, run it, and found something that surprise you, reevaluate your assumptions, be willing to debug it#. Don’t just “know” your code works, prove it, in this context, with this data, with these boundary conditions. Add new test# to it, Crash Program Earlier# or put some Assertions#. You can go further by adopting Defensive Programming#, enforcing checks on precondition, postcondition and class invariant by Design by Contract (DBC)#.