Root Causes of Unstable Test Cases

Automated testing can become unreliable when test cases are flaky, and understanding the root causes is essential for maintaining development velocity.

The instability originates from four layers: the test case itself, the test execution framework, the system under test (including its dependent services), and the underlying operating system and hardware.

Test case–related causes include improper setup or cleanup, invalid assumptions about test data or system state (e.g., using system time), timing dependencies, and reliance on execution order.

Setup or cleanup issues

Invalid test‑data assumptions

Invalid system‑state assumptions

Timing‑sensitive tests

Order‑dependent tests

Framework‑related causes involve insufficient resources allocated to the system under test, poorly planned test schedules that cause test conflicts, and overall resource shortages.

Insufficient resources for the SUT

Conflicting test plans

General resource constraints

System‑under‑test and dependency causes cover race conditions, uninitialized variables, slow or unresponsive service calls, memory leaks, over‑provisioned resources, and mismatched response expectations.

Race conditions

Uninitialized variables

Slow or missing responses

Memory leaks

Resource overallocation

Unexpected response latency

Operating system and hardware causes include network failures or instability, disk errors, and unrelated background tasks or services consuming resources needed by the tests.

Network failures/instability

Disk errors

Resource‑hungry unrelated tasks

Using a hermetic (self‑contained) test environment greatly reduces these sources of flakiness, but keeping the instability rate low remains a significant challenge.

The article provides this classification as a reference for developers to diagnose and mitigate flaky tests, with a follow‑up article promised to discuss remediation strategies.