Ask HN: What are the points to remember while doing code review?

I am working on project which has thousands of files and millions of lines of code. Also, we are constantly updating it with new enhancements and bug fixes.

Now, since last few months it is happening that new enhancements happen over the system and it brake something.

I know it is impossible to eliminate error but what are the points or things or strategy that you follow while doing code review that help to reduce these types of broken effects.