I&#x27;ve worked on projects before which initially had no tests, then some tests, then progressively more and more sophisticated tests.<p>Each time the harness became capable enough to replicate a certain type of scenario it didn&#x27;t take long before those kinds of bugs dried up.<p>The bugs then always nearly always migrated to the areas the test harness couldn&#x27;t easily create scenarios for - whether that was interactions with crazy external APIs, odd timezones or weird quirky browsers or whatever.<p>Continually modifying the test harness to be made capable of testing bugs reported from production was often a huge amount of work but it paid off handsomely.