Developer finds data-drift detector crying wolf due to floating-point and HLL noise
A developer building a pipeline drift-detection tool designed a benchmark of 56 seeded faults, which the tool resolved correctly in 55 out of 56 cases. To check for false positives, four negative control scenarios were added where the correct detector response was silence. The null control — a run with no data changes at all — failed, generating three high-severity alerts. Investigation revealed two root causes: HyperLogLog's probabilistic distinct-count estimates varied by up to 30% between identical runs, and DuckDB's parallel aggregation produced non-deterministic floating-point results that made identical values appear different. The fix was to use exact distinct counts and round floats before comparison, underscoring that measurement determinism is a prerequisite for reliable anomaly detection.
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