You can't design a perfect system that accounts for every possibility or handles every edge case.
Assuming everything will work as expected is a mistake. Failure is inevitable in any system.
A bad connection, a slow database, or an unavailable third-party dependency can badly affect your system if you don't handle it correctly. Design your system around what happens when things go wrong. Plan for retries, fallbacks, and recovery.
Otherwise, a small failure can quickly become a much bigger problem.
A resilient system isn't one that never fails. It's one that knows how to recover when it does.