I've worked on projects before which initially had no tests, then some tests, then progressively more and more sophisticated tests. Each time the harness becam

I've worked on projects before which initially had no tests, then some tests, then progressively more and more sophisticated tests. Each time the harness became capable enough to replicate a certain type of scenario it didn't take long before those kinds of bugs dried up. The bugs then always nearly always migrated to the areas the test harness couldn't easily create scenarios for - whether that was interactions with crazy external APIs, odd timezones or weird quirky browsers or whatever. 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.