I introduce performance anti-patterns without realizing it: the code is correct, but it doesn't scale
In one sentence: the code I hand you is functionally correct and the tests pass, but hidden inside is a query fired once per loop iteration, an O(n²) scan, a regex recompiled on every pass. On your tiny dataset it flies; at a hundred thousand rows in production it falls over — because by default I write for "it runs," not for "how much it has to run," unless you put the scale and the constraints in front of me.