I’m not the greatest QA tester. I’m not even sure if you’d consider me mid-level.
But I know the results of what happens when you don’t test properly when all you focus on is the happy path and never consider what might go wrong.
Edge cases aren’t my master but they do sit at the back of my mind, driving the course of development so I can be sure they are accounted for in what I deliver. I might screw them up the first few times, but when I go to the demo, they would have been thought of and taken into consideration into the design before me.
You don’t need to account for all of them, some aren’t worth it, but if all you’re focused on is the happy path, the low volume path – that’s all your code will ever be.