I've been working at Read the Docs for four months more or less at this time. Although I've been using Django for a decade now, there are many things that I never used, forget or don't know how they work internally.
During the last three or four weeks I worked in a new feature for the corporate site of Read the Docs (https://readthedocs.com/) and I had a lot of different problems: mainly because I didn't know the full codebase, but also because I never used some pieces of technology involved to make this feature.
So, what do you do when you are afraid of breaking everything? You write many test to cover your ass! That's what I did and I felt very comfortable when I started writing the logic of the feature itself --of course, to write the tests I had to ask many questions to my team to be sure the tests were accurate regarding what they were testing.
Finally, the PR was tested for some members of the team, everything worked as expected, the code was merged and deployed. Successful story, right?
One of two days later, I started seeing one of my tests related to that PR failing locally. I did a simple PR to run the test in CircleCI and it also failed there. "WTF? This doesn't make sense" -I thought. I did the manual QA locally and the code worked as expected but the test were failing but... It didn't make sense because the initial tests under CircleCI before merging the PR were passing and now without any change at all this specific test wasn't passing. I was very confused.