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C'mon, guys! How long have memory leaks been known? Why are people still coding them?
That's like saying "How long have car crashes been known? Why are people still having them?"
When working on a project that requires a large team, no matter how hard you try, there will be occasions where a person doesn't realize the implications of what they're doing on what other people are doing. You can't avoid problems, you can only try to minimize how often they happen.