Testing the untestable: navigating edge cases in legacy public sector systems

Legacy systems are unpredictable by nature. Missing documentation, outdated integrations, and bespoke business rules mean surprises are inevitable. Standard testing plans often focus on the “happy path,” which only confirms that everything works when conditions are ideal, leaving edge cases - rare scenarios that surface under unusual conditions - undetected.

From my experience leading complex migration testing projects in the public sector, identifying these hidden behaviours early is critical - and this is where structured QA, combined with deep business knowledge, makes all the difference. QA can’t be an afterthought in public-sector migrations – as I explored in a previous post.

Finding edge cases before they cause a problem

The most effective migrations are those that identify potential problems before they appear in production. One way to do this is through discovery workshops. Bring QA, developers, and service owners together to map end-to-end workflows - these sessions often reveal unusual processes and hidden exceptions.

Another key tool is data profiling. By examining historical patterns and anomalies, you can anticipate behaviours that only appear in rare circumstances. Complementing this with exploratory testing - where testers “poke at the edges” of the system - provides a deeper understanding of potential risks.

The real value emerges when technical QA is paired with deep business knowledge. Understanding not just what could go wrong, but why, surfaces hidden behaviours early and prevents surprises at cutover.

Designing tests for the unexpected 

The challenge is designing tests that can safely simulate and validate edge cases. Synthetic datasets allow safe replication of rare or extreme conditions, while scenario modelling simulates real-world triggers such as unusual user actions, atypical transaction sequences, or legacy integrations behaving differently under load.

Flexible, automated test suites are another cornerstone of this approach. As new exceptions are discovered, these suites can be updated and rerun quickly, ensuring fixes don’t introduce new issues. By combining adaptability with automation, QA turns uncertainty into a controlled process, ensuring even complex edge cases are covered.

Collaboration as a safeguard

QA doesn’t exist in isolation; it’s most effective when integrated across the whole project. Shared visibility into test results allows testers, developers, and service owners to spot anomalies in real time, rather than waiting until final cutover.

For example, in our work with Southern Housing, daily collaboration between project managers, developers, and our testers - through stand-ups, agile ceremonies, and planning meetings - enabled early detection and resolution of potential issues.

Transparency and open communication turn QA from a reactive checkpoint into a proactive safeguard, preparing your team to anticipate problems and adapt to unexpected behaviours.

Turning risk into readiness

Edge cases are inevitable - but they don’t have to be risky. Continuous QA, early identification of potential problems, validation of data transformations, and monitoring can create systems that are resilient, adaptable, and trustworthy.

QA transforms uncertainty into understanding, ensuring public sector modernisation isn’t just about moving data or upgrading technology - it’s about delivering a reliable, citizen-focused transition.

Your six key takeaways

1. Edge cases hide the biggest risks - early QA brings them to light.
2. Discovery workshops and data profiling uncover hidden behaviours.
3. Scenario-based testing ensures rare workflows function correctly.
4. Synthetic data and flexible automation support safe, repeatable testing.
5. Collaboration across teams turns findings into actionable insight.
6. Continuous QA creates systems that are robust, reliable, and ready for anything.

About Zoonou

Zoonou is a UK-based digital QA company. We’re a B Corp and 100% employee owned. We collaborate with public sector organisations to help build digital services that work for the people who use them and support the teams who create them - driving better outcomes for citizens and businesses.