How a Simple Pre‑processing Order Leak Inflated a Car‑Price Model’s R² by 12 Points
A car‑price regression model reported an R² of 0.887, but a two‑line preprocessing mix‑up let the test set leak into training, inflating the score by 0.12. Fixing the order dropped R² to 0.767, exposing the hidden leakage. The post shows a concrete, easy‑to‑miss data‑leak pitfall and how to catch it.