Classic examples are batches of near-identical pages targeting many cities or keyword variants, all funnelling to the same place, or pages that exist only to catch traffic and push users somewhere they didn't intend to go. The tell is that the page serves the ranking, not the user.
This matters because the line between a legitimate local landing page and a doorway page is real but not always obvious. Genuinely useful, distinct location or service pages — with unique content and real value — are fine. Thin, templated duplicates that add nothing are the risk, and they can draw an algorithmic filter or a manual action.
The safe approach is to build fewer, genuinely valuable pages, each serving a clear intent. If you're tempted to spin up dozens of near-duplicates, that's usually a sign to consolidate instead — which also avoids cannibalization. Spotting doorway risk is a standard part of an SEO audit.