Though Architecture-developer-test-Tech Support teams are responsible for the Leaked defect, most of the companies have a ingrained culture of blaming the testers.

I had been in situation where the APIs were continusouly changed on a regular basis and  there was no coordinated communication between the teams involved.  Testers were continously changing the API test framework, which resulted missing out some of the scenarios which resulted in leak defects in the System Integration testing.

RCA was done badly, we just found the gaps in our test design & how to improve the test design, but never focussed on why we missed them during our test design 

After taking the entire blame, we started setting up rules for our test team.

1. Feature Interaction matrix for all the APIs with respect to their interactions 

2. Never change the Test design unless there is a published(approved by Dev. manager/Architect in consultation with Test ) API/Feature change and change should come through a Single Point of Contact.

3. Test Reviews to be done diligently and test team should discuss more with in ourselves as well as Dev. team on the reason behind the changes and what type of problems can arise due to the changes.