The one interview question that exposed I didn't actually understand hash maps, just memorized the syntax

Could implement one from memory, knew the average time complexity cold, aced every practice problem that used one. Then an interviewer asked what happens to that complexity under a bad hash function, and I just froze — never actually thought about collisions past the abstract idea that they exist. Went and read about load factor and resizing that night. Passing practice problems isn't the same as understanding the thing.