{"abstract":"Two updates calculated from the same snapshot leave only one increment.","category":"Concurrency","checks":3,"contract":"Apply each delta to the current accumulator rather than replacing it from a stale base.","evaluation_group":"model-acda882c06bc5737","failed_approach":"Deduplicating equal deltas mistakes independent operations for retries.","family":"lost-update","id":"FA-016","implementations":{"attempt":{"sha256":"7a31b49393f5f97d163df4a28aadf3b70940d341eafe59da23b4f75aad144ed9","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(base, deltas):\n    return base + sum(set(deltas))\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('two independent updates', solve(N, [2, 2]), N+4)\ncheck('mixed deltas', solve(N, [1, -1, 3]), N+3)\ncheck('no updates', solve(N, []), N)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"broken":{"sha256":"3cb19adfe6d019c217104ee3e2bc1a5304db9d3c3f8c6b0c680c0689a123ba2d","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(base, deltas):\n    return base + deltas[-1] if deltas else base\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('two independent updates', solve(N, [2, 2]), N+4)\ncheck('mixed deltas', solve(N, [1, -1, 3]), N+3)\ncheck('no updates', solve(N, []), N)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"fixed":{"sha256":"6b6762836483a1fcc77f02d4b453abb78347a02cb7470f9026fab1911895b4e2","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(base, deltas):\n    return base + sum(deltas)\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('two independent updates', solve(N, [2, 2]), N+4)\ncheck('mixed deltas', solve(N, [1, -1, 3]), N+3)\ncheck('no updates', solve(N, []), N)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"}},"limitations":" This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.","method":"Deterministic executable model with adversarial boundary fixtures.","provenance":{"created_by":"Failure Map","dependencies":"Python standard library","family":"lost-update","generated_at":"2026-09-29T14:36:49.861722+00:00","license":"CC0-1.0","python":"3.12.14","seed":1,"split":"open-access"},"relevance":"A compact reasoning and repair task with executable evidence.","repair":"Apply each delta to the current accumulator rather than replacing it from a stale base.","root_cause":"Absolute values calculated from a stale base replace each other.","sha256":"a9068f0b8bfe1d0684ddc19163ba50fd472eda5f439d5567b15c587a8e2866af","title":"Stale snapshots erase a counter increment · case 01","variant":1,"variant_policy":"Five numbered records share a model and may reuse boundary fixtures.","verification":{"attempt":{"elapsed_ms":32.211,"exit_code":1,"observations":[{"actual":3,"check":"two independent updates","expected":5,"passed":false},{"actual":4,"check":"mixed deltas","expected":4,"passed":true},{"actual":1,"check":"no updates","expected":1,"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"two independent updates\", \"actual\": 3, \"expected\": 5, \"passed\": false}, {\"check\": \"mixed deltas\", \"actual\": 4, \"expected\": 4, \"passed\": true}, {\"check\": \"no updates\", \"actual\": 1, \"expected\": 1, \"passed\": true}], \"passed\": false}\n"},"broken":{"elapsed_ms":38.417,"exit_code":1,"observations":[{"actual":3,"check":"two independent updates","expected":5,"passed":false},{"actual":4,"check":"mixed deltas","expected":4,"passed":true},{"actual":1,"check":"no updates","expected":1,"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"two independent updates\", \"actual\": 3, \"expected\": 5, \"passed\": false}, {\"check\": \"mixed deltas\", \"actual\": 4, \"expected\": 4, \"passed\": true}, {\"check\": \"no updates\", \"actual\": 1, \"expected\": 1, \"passed\": true}], \"passed\": false}\n"},"fixed":{"elapsed_ms":34.072,"exit_code":0,"observations":[{"actual":5,"check":"two independent updates","expected":5,"passed":true},{"actual":4,"check":"mixed deltas","expected":4,"passed":true},{"actual":1,"check":"no updates","expected":1,"passed":true}],"passed":true,"stderr":"","stdout":"{\"observations\": [{\"check\": \"two independent updates\", \"actual\": 5, \"expected\": 5, \"passed\": true}, {\"check\": \"mixed deltas\", \"actual\": 4, \"expected\": 4, \"passed\": true}, {\"check\": \"no updates\", \"actual\": 1, \"expected\": 1, \"passed\": true}], \"passed\": true}\n"}},"verified":true,"visibility":"public"}