In KiCad a custom .kicad_dru rule overrides the board and every net class: the Sierra fab rule set (4 mil clearance, 6 mil hole and ring) pulled 0.2 mm and 0.15 mm net-class clearances down to 4 mil, and pours filled 0.1 mm from pads, so the Last Hit v0.12 board passed DRC that way, while a copy without the rule file had 436 errors. Fab rules are meant to add floors, never loosen. Logged as mistake M62.