A bank wants one fraud model across three systems it acquired. Each one labeled fraud differently. Scope the first 90 days.

Candidates reliably identify entity resolution because it is visible in the schema. Almost nobody interrogates the label, which is invisible: each bank had its own confirmation process, its own write-off threshold, and its own lag between a transaction and a chargeback, so a 1 in one system is not a 1 in another. Training on the union produces a model whose errors are systematically worse for whichever bank had the strictest definition, and nobody finds out until it declines that bank's customers. Naming this before being prompted is what separates the strong answers.